diff --git a/src/matcalc/__init__.py b/src/matcalc/__init__.py
index de0b8ce..1f0564b 100644
--- a/src/matcalc/__init__.py
+++ b/src/matcalc/__init__.py
@@ -13,6 +13,7 @@
 from ._base import ChainedCalc, PropCalc
 from ._elasticity import ElasticityCalc
 from ._eos import EOSCalc
+from ._gb import GBCalc
 from ._interface import InterfaceCalc
 from ._lammps import LAMMPSMDCalc
 from ._md import MDCalc
diff --git a/src/matcalc/_gb.py b/src/matcalc/_gb.py
new file mode 100644
index 0000000..6da0f40
--- /dev/null
+++ b/src/matcalc/_gb.py
@@ -0,0 +1,292 @@
+"""Grain Boundary Energy calculations."""
+
+from __future__ import annotations
+
+import logging
+import math
+from typing import TYPE_CHECKING, Any
+
+import numpy as np
+from pymatgen.core.interface import GrainBoundaryGenerator
+from pymatgen.symmetry.analyzer import SpacegroupAnalyzer
+
+from ._base import PropCalc
+from ._relaxation import RelaxCalc
+from .utils import to_pmg_structure
+
+if TYPE_CHECKING:
+    from ase import Atoms
+    from ase.calculators.calculator import Calculator
+    from ase.optimize.optimize import Optimizer
+    from pymatgen.core import Structure
+
+
+class GBCalc(PropCalc):
+    """
+    A class for performing grain boundary energy calculations by generating grain boundary structures
+    from a bulk structure using pymatgen's GrainBoundaryGenerator, optionally relaxing bulk and GB,
+    and computing γ_GB via (E_GB - N_GB * E_bulk_per_atom) / (2 * A).
+
+    :ivar calculator: ASE Calculator used for energy and force evaluations.
+    :type calculator: Calculator
+
+    :ivar relax_bulk: Whether to relax the bulk structure (cell and atoms) before GB generation.
+    :type relax_bulk: bool
+
+    :ivar relax_gb: Whether to relax the grain boundary structures (atoms only) after generation.
+    :type relax_gb: bool
+
+    :ivar fmax: Force convergence criterion (eV/Å) for relaxations.
+    :type fmax: float
+
+    :ivar optimizer: Optimizer for structure relaxations.
+    :type optimizer: str | Optimizer
+
+    :ivar max_steps: Maximum optimization steps for relaxations.
+    :type max_steps: int
+
+    :ivar relax_calc_kwargs: Additional keyword arguments for relaxation calculations. Defaults to None.
+    :type relax_calc_kwargs: dict | None
+    """
+
+    def __init__(
+        self,
+        calculator: Calculator | str,
+        *,
+        relax_bulk: bool = True,
+        relax_gb: bool = True,
+        fmax: float = 0.1,
+        optimizer: str | Optimizer = "FIRE",
+        max_steps: int = 500,
+        relax_calc_kwargs: dict | None = None,
+    ) -> None:
+        """
+        Constructor for initializing the GBCalc with all parameters needed
+        to generate and optionally relax bulk and grain boundary structures.
+
+        :param calculator: ASE Calculator or string identifier for a universal calculator.
+        :type calculator: Calculator | str
+
+        :param relax_bulk: Whether to relax the bulk structure before GB generation. Default True.
+        :type relax_bulk: bool, optional
+
+        :param relax_gb: Whether to relax the GB structures after generation. Default True.
+        :type relax_gb: bool, optional
+
+        :param fmax: Force tolerance (eV/Å) for relaxations. Default 0.1.
+        :type fmax: float, optional
+
+        :param optimizer: ASE optimizer or name for relaxations. Default "FIRE".
+        :type optimizer: str | Optimizer, optional
+
+        :param max_steps: Max optimization steps. Default 500.
+        :type max_steps: int, optional
+
+        :param relax_calc_kwargs: Additional keyword arguments for relaxation calculations. Defaults to None.
+        :type relax_calc_kwargs: dict | None
+        """
+        self.calculator = calculator  # type: ignore[assignment]
+        self.relax_bulk = relax_bulk
+        self.relax_gb = relax_gb
+        self.fmax = fmax
+        self.optimizer = optimizer
+        self.max_steps = max_steps
+        self.relax_calc_kwargs = relax_calc_kwargs
+
+    def calc_gb(
+        self,
+        structure: Structure | Atoms,
+        sigma: int,
+        rotation_axis: tuple[int, int, int],
+        gb_plane: tuple[int, int, int],
+        rotation_angle: float,
+        expand_times: int = 4,
+        vacuum_thickness: float = 0.0,
+        ab_shift: tuple[float, float] = (0.0, 0.0),
+        rotation_angle_tolerance: float = 0.1,
+        normal: bool = True,
+        rm_ratio: float = 0.7,
+        gb_generator_kwargs: dict | None = None,
+        gb_from_parameters_kwargs: dict | None = None,
+    ) -> dict[str, Any]:
+        """
+        Generate grain boundary structures from bulk, relax them, and compute GB energies.
+
+        :param structure: Bulk Structure for GB generation.
+        :type structure: Structure
+
+        :param rotation_axis: Rotation axis vector for GB (u, v, w).
+        :type rotation_axis: tuple[int, int, int]
+
+        :param gb_plane: Miller indices of GB plane.
+        :type gb_plane: tuple[int, int, int]
+
+        :param rotation_angle: Rotation angle in degrees.
+        :type rotation_angle: float
+
+        :param expand_times: Multiplier for cell expansion to avoid interactions. Default 4.
+        :type expand_times: int, optional
+
+        :param vacuum_thickness: Vacuum layer thickness (Å) between grains. Default 0.0.
+        :type vacuum_thickness: float, optional
+
+        :param ab_shift: In-plane shift along a, b vectors. Default (0.0, 0.0).
+        :type ab_shift: tuple[float, float], optional
+
+        :param normal: Determine if the c axis of top grain should perpendicular to the surface. Default True.
+        :type normal: bool, optional
+
+        :param rm_ratio: The criteria to remove the atoms which are too close with each other. Default 0.7.
+        :type rm_ratio: float, optional
+
+        :param gb_generator_kwargs: Additional args for GrainBoundaryGenerator(). Default None.
+        :type gb_generator_kwargs: dict | None, optional
+
+        :param gb_from_parameters_kwargs: Additional args for gb_from_parameters(). Default None.
+        :type gb_from_parameters_kwargs: dict | None, optional
+
+        :return:  A dictionary containing the updated structure data, including fields like
+                 'grain_boundary', 'final_grain_boundary', 'gb_relax_energy', 'grain_boundary_energy', and possibly updated
+                 'bulk_energy_per_atom' and 'final_bulk'.
+        :rtype: dict[str, Any]
+        """
+        # GB generation - start with a primitive structure.
+        structure = to_pmg_structure(structure)
+        bulk_primitive = structure.to_primitive()
+        bulk_conventional = structure.to_conventional()
+
+        # Bulk relaxation
+        relax_bulk_calc = RelaxCalc(
+            calculator=self.calculator,
+            fmax=self.fmax,
+            max_steps=self.max_steps,
+            relax_cell=self.relax_bulk,
+            relax_atoms=self.relax_bulk,
+            optimizer=self.optimizer,
+            **(self.relax_calc_kwargs or {}),
+        )
+
+        bulk_opt = relax_bulk_calc.calc(bulk_conventional)
+        final_bulk = bulk_opt["final_structure"]
+        bulk_energy_per_atom = bulk_opt["energy"] / len(final_bulk)
+
+        gb_gen = GrainBoundaryGenerator(
+            initial_structure=bulk_primitive,
+            **(gb_generator_kwargs or {}),
+        )
+
+        analyzer = SpacegroupAnalyzer(bulk_primitive)
+        lattice_type = analyzer.get_crystal_system()[0]
+        c_a_ratio = gb_gen.get_ratio()
+        angles = gb_gen.get_rotation_angle_from_sigma(
+            sigma=sigma, r_axis=rotation_axis, lat_type=lattice_type, ratio=c_a_ratio
+        )
+
+        # look through the list of computed angles and find the one
+        # that is within the tolerance of the requested angle
+        rotation_angle_exact = next(
+            (a for a in angles if math.isclose(a, rotation_angle, abs_tol=rotation_angle_tolerance)),
+            None,
+        )
+        # …and if none are within the tolerance, error out
+        if rotation_angle_exact is None:
+            raise ValueError(
+                f"No matching rotation angle {rotation_angle} for sigma={sigma}. \
+                                Possible angles: {angles}"
+            )
+
+        grain_boundary = gb_gen.gb_from_parameters(
+            rotation_axis=rotation_axis,
+            rotation_angle=rotation_angle_exact,
+            plane=gb_plane,
+            expand_times=expand_times,
+            vacuum_thickness=vacuum_thickness,
+            ab_shift=ab_shift,
+            normal=normal,
+            rm_ratio=rm_ratio,
+            **(gb_from_parameters_kwargs or {}),
+        )
+
+        return self.calc(
+            {
+                "grain_boundary": grain_boundary,
+                "bulk_energy_per_atom": bulk_energy_per_atom,
+                "final_bulk": final_bulk,
+            }
+        )
+
+    def calc(
+        self,
+        structure: Structure | Atoms | dict[str, Any],
+    ) -> dict[str, Any]:
+        """
+        Compute grain boundary energy for a single grain boundary structure dict produced by calc_gbs or direct input.
+        The function is equipped to handle the relaxation of both bulk and grain boundary structures when necessary.
+
+        :param structure: Dictionary containing information about the bulk and grain boundary structures.
+                          It must have the format:
+                          {'grain_boundary': gb_structure, 'bulk': bulk_structure}
+                          or
+                          {'grain_boundary': gb_structure, 'bulk_energy_per_atom': energy}.
+        :type structure: Structure | Atoms | dict[str, Any]
+
+        :return:  A dictionary containing the updated structure data, including fields like
+                 'grain_boundary', 'final_grain_boundary', 'gb_relax_energy', 'grain_boundary_energy', and possibly updated
+                 'bulk_energy_per_atom' and 'final_bulk'.
+        :rtype: dict[str, Any]
+        """
+        if not (isinstance(structure, dict) and set(structure.keys()).intersection(("bulk", "bulk_energy_per_atom"))):
+            raise ValueError(
+                "For grain boundary calculations, structure must be a dict in one of the following formats: "
+                "{'grain_boundary': gb_structure, 'bulk': bulk_structure} or {'grain_boundary': gb_structure, 'bulk_energy_per_atom': energy}."
+            )
+
+        result_dict = structure.copy()
+
+        if "bulk_energy_per_atom" in structure:
+            bulk_energy_per_atom = structure["bulk_energy_per_atom"]
+        else:
+            logging.info("Relaxing bulk structure with both atomic and cell degrees of freedom")
+            relaxer = RelaxCalc(
+                calculator=self.calculator,
+                fmax=self.fmax,
+                max_steps=self.max_steps,
+                relax_cell=self.relax_bulk,
+                relax_atoms=self.relax_bulk,
+                optimizer=self.optimizer,
+                **(self.relax_calc_kwargs or {}),
+            )
+            bulk_opt = relaxer.calc(structure["bulk"])
+            final_bulk = bulk_opt["final_structure"]
+            bulk_energy_per_atom = bulk_opt["energy"] / len(final_bulk)
+            result_dict["bulk_energy_per_atom"] = bulk_energy_per_atom
+            result_dict["final_bulk"] = final_bulk
+
+        # Relax GB
+        gb_struct = structure["grain_boundary"]
+        relax_gb_calc = RelaxCalc(
+            calculator=self.calculator,
+            fmax=self.fmax,
+            max_steps=self.max_steps,
+            relax_cell=False,
+            relax_atoms=self.relax_gb,
+            optimizer=self.optimizer,
+            **(self.relax_calc_kwargs or {}),
+        )
+        gb_opt = relax_gb_calc.calc(gb_struct)
+        final_gb = gb_opt["final_structure"]
+        gb_energy = gb_opt["energy"]
+
+        # Compute area (a * b)
+        lattice = final_gb.lattice.matrix
+        area = np.linalg.norm(np.cross(lattice[0], lattice[1]))
+        n_atoms = len(final_gb)
+
+        # Compute grain boundary energy
+        gamma_gb = (gb_energy - n_atoms * bulk_energy_per_atom) / (2 * area)
+
+        return result_dict | {
+            "final_grain_boundary": final_gb,
+            "gb_relax_energy": gb_energy,
+            "grain_boundary_energy": gamma_gb,
+        }
diff --git a/tests/test_gb.py b/tests/test_gb.py
new file mode 100644
index 0000000..2243ea6
--- /dev/null
+++ b/tests/test_gb.py
@@ -0,0 +1,75 @@
+"""Tests for the GBCalc class."""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+import pytest
+
+from matcalc import GBCalc
+
+if TYPE_CHECKING:
+    from matgl.ext.ase import PESCalculator
+    from pymatgen.core import Structure
+
+
+def test_GB_calc_basic(Si: Structure, m3gnet_calculator: PESCalculator) -> None:
+    """
+    Test the basic workflow:
+      1) calc_gbs on a known structure
+      2) Check the final results
+    """
+    gb_calc = GBCalc(
+        calculator=m3gnet_calculator,
+        relax_bulk=True,
+        relax_gb=True,
+        fmax=0.1,
+        max_steps=100,
+    )
+
+    results = gb_calc.calc_gb(
+        Si,
+        sigma=1,
+        rotation_axis=(1, 1, 1),
+        gb_plane=(1, 1, 1),
+        rotation_angle=0.0,
+        expand_times=1,
+    )
+
+    assert "final_bulk" in results
+    assert "final_grain_boundary" in results
+    assert "gb_relax_energy" in results
+    assert "grain_boundary_energy" in results
+    assert "bulk_energy_per_atom" in results
+
+    assert results["bulk_energy_per_atom"] == pytest.approx(ABC, rel=1e-1)
+    assert results["gb_relax_energy"] == pytest.approx(ABC, rel=1e-1)
+    assert results["grain_boundary_energy"] == pytest.approx(ABC, rel=1e-1)
+
+    results = gb_calc.calc({"grain_boundary": Si, "bulk": Si})
+
+    assert "final_bulk" in results
+    assert "final_grain_boundary" in results
+    assert "gb_relax_energy" in results
+    assert "grain_boundary_energy" in results
+    assert "bulk_energy_per_atom" in results
+
+    assert results["bulk_energy_per_atom"] == pytest.approx(ABC, rel=1e-1)
+    assert results["gb_relax_energy"] == pytest.approx(ABC, rel=1e-1)
+    assert results["grain_boundary_energy"] == pytest.approx(ABC, rel=1e-1)
+
+
+def test_surface_calc_invalid_input(Si: Structure, m3gnet_calculator: PESCalculator) -> None:
+    """
+    If the user passes a non-dict to calc, it should raise ValueError.
+    """
+    surf_calc = GBCalc(calculator=m3gnet_calculator)
+    with pytest.raises(
+        ValueError,
+        match="For grain boundary calculations, structure must be a dict in one of the following formats:",
+    ):
+        surf_calc.calc(Si)
+    with pytest.raises(
+        ValueError, match="For grain boundary calculations, structure must be a dict in one of the following formats:"
+    ):
+        surf_calc.calc({"grain_boundary": Si})