Thermal expansion of β-Mg₂SiO₄ (modified spinel; orthorhombic symmetry) was measured by the X-ray powder diffraction method in the temperature range between 20°C and 800°C. Grüneisen's theory of thermal expansion was applied to the linear and volume expansion data, and thermal expansion coefficients were obtained to be α_a=9.14, α_b=5.46, α_c=5.92, and α_v=20.6 in unit of 10^-6/K, at 293K. Gruneisen's parameter γ_G=1.26 was obtained by assuming the Debye temperature to be 952K. The magnitude of volume expansion coefficient of β-Mg₂SiO₄ is located between those of γ- and α-phases. β-Mg₂SiO₄ is found to be a significantly anisotropic material both in thermal expansion and elasticity.