Walstromite-structured CaSiO3 (Wal) was synthesized at 6 GPa and 1200 °C for 6 h using a cubic press, and its thermal elastic behavior was investigated at T up to 900 °C using a powder X-ray diffraction technique at ambient pressure. Within the investigated T range, all unit-cell parameters, j, of Wal varied almost linearly with T, so that we fitted the data with the equation αj = j−1(∂j/∂T) and obtained αa = 0.92(2) × 10−5/°C, αb = 1.65(1) × 10−5/°C, αc = 0.83(1) × 10−5/°C, and αV = 3.24(3) × 10−5/°C for Wal. The magnitudes of the principal Lagrangian strain coefficients (ɛ1, ɛ2, and ɛ3) and the orientation of the thermal strain ellipsoids, between ambient T and measured T, were calculated. The orientation of the strain ellipsoid appears constant with T variation, whereas the strain magnitudes vary significantly with T: ɛ1 increases, but ɛ2 and ɛ3 decrease. For T > 900 °C, primitive data were collected for “parawollastonite” (Wo-2M), which led to a much smaller volumetric thermal expansion coefficient than that of Wal.