Thermal expansion of γ-Mg₂SiO₄ was measured with X-ray powder diffraction method up to 750°C at atmospheric pressure. The data are analyzed in terms of Gruneisen's theory of thermal expansion and Gruneisen's parameter is obtained to be 1.27. Volume thermal expansion coefficient is 19 at 20°C and 27 at 500°C in unit of 10^-6/K. These values are compared with those of γ-Fe₂SiO₄, MgAl₂O₄ and other spinel type compounds, and it is suggested that the anharmonic properties of γ-(Mg, Fe)₂SiO₄ may be very close to those of MgAl₂O₄.