The volume of MgSiO3 perovskite has been precisely measured at pressures of 19 to 53 GPa and temperatures of 300 to 2300 K by means of in situ X‐ray diffraction in a multi‐anvil apparatus. The present results indicate the isothermal bulk modulus KT0 = 256(2) GPa and its pressure derivative K′T0 = 3.8(2). The fixed Debye temperature θ0 = 1030 K gives a Grüneisen parameter at ambient pressure γ0 = 2.6(1) and its logarithmic volume dependence q = 1.7(1). The pressure derivative of the isothermal bulk modulus, Anderson‐Grüneisen parameter and thermal expansion coefficient at ambient pressure are found to be (∂KT/∂T)P = −0.035(2) GPa/K, δT = 6.5(5), α0 = 2.6(1) × 10−5 + 1.0(1) × 10−8 (T − 300)/K. Thus the thermal expansion coefficient largely becomes smaller with increasing pressure. The adiabatic geotherm would be fairly large, such as 0.41 K/km at a 660 km depth, and becoming smaller with increasing depth. The temperature and adiabatic geothermal gradient at the bottom of the D′ layer would be 2400 K and 0.14 K/km. The buoyancy‐driven mantle convection could be very small in the lower part of the lower mantle.

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