In this study, crystals of Y2SiO5 were examined by high-temperature powder x-ray diffractometry to determine the changes in unit-cell dimensions with temperatures up to 1273 K for the X1 phase (the low-temperature phase, space group P121/c1) and 1473 K for the X2 phase (the high-temperature phase, space group I12/a1). The lattice deformations of both phases induced by thermal expansion were investigated by matrix algebra analysis to determine the directions and magnitudes of the principal distortions (λi, i = 1, 2, and 3). For the X1 phase, λ1 and λ2 invariably showed a positive thermal expansion. On the other hand, λ3 showed a negative thermal expansion below 1173 K; the maximum contraction of 0.10(4)% occurred at 685 K. The λ2 axis invariably coincides with the crystallographic b axis. The directions of λ1 and λ3, defined by the acute angle λ3 ^ c changed between 53(3)° (T = 394 K) and 45(1)° (T = 788 K). For the X2 phase, all of the principal distortions steadily increased with increasing temperature. The angle λ3 ^ c steadily decreased from 71(2)° to 62.1(1)° with increasing temperature. The mean linear thermal expansion coefficients were, when compared at the same temperatures, necessarily higher for the X1 phase than for the X2 phase. The lattice change of X1–RE2SiO5 (RE = Y and Yb–La), which was induced by the substitution of rare-earth (RE) ions, showed a striking resemblance with the lattice deformation of X1-Y2SiO5, which was caused by the thermal expansion. Because the lattice change of the former must be caused by the isotropic expansion of the RE sites, the anisotropic thermal expansion of the latter would be essentially attributable to the isotropic thermal expansion of the YO9 and YO7 polyhedron.