In situ X ray experiments on MgSiO3 perovskite at pressures of 21--29 GPa and temperatures of 300--2000 K were carried out using an MA8-type high-pressure apparatus, employing sintered diamond anvils, combined with synchrotron radiation. The thermal expansion at 25 GPa up to 2000 K was determined from interpolation of the P-V-T data obtained in the present study. The 95% confidence level was estimated by taking all possible errors into account, including statistical error and systematic error caused by uncertainty of pressure scales, etc. The thermal expansivity at 25 GPa is expressed as αT,25=a25+b25T-c25T2 with the bestfit parameters of a25=2.11¿10-5 K-1, b25=1.80¿10-9 K-2, and c25=1.93 K. The equation of state of MgSiO3 perovskite has been determined using our new data combined with the lower-pressure data of Wang et al. <1994> and Utsumi et al. <1995>. The optimal set of parameters of the third-order Birch-Murnaghan equation of state, which is expressed as P=(3/2)KT,0<(VT,0/V)7/3-(VT,0/V)5/3>{1-(3/4)(4-KT,0 ') ¿<(VT,0/V)2/3-1>}, where KT,0=K300,0+(∂KT,0/∂T)P(T-300), KT,0'=K300,0', VT,0=V0 exp ∫T300αT,0dT, and αT,0=a0+b0T-c0T-2, is K300,0=261 GPa, K300,0'=4, a0=1.982¿10-5 K-1, b0=0.818¿10-8 K-2, c0=0.474 K, and (∂KT,0/∂T)P=-0.0280 GPa/K. The reliability of the result is discussed in detail. ¿ American Geophysical Union 1996