A double-stage multianvil system was developed to conduct in situ X ray diffraction study at high pressures. The system generates pressures and temperature above 25 GPa and 1200 ¿ and has been used in conjunction with intense synchrotron radiation to study the phase transitions of MgSiO3 and the thermal expansion of MgSiO3 perovskite. At pressures between 20 and 30 GPa, we observed a series of phase transitions from spinel+stishovite through ilmenite to perovskite. Spinel+stishovite transforms into ilmenite at 20.0¿1.0 GPa, and ilmenite transforms into perovskite at 24.0¿0.5 GPa at 800 ¿C. The phase boundary between ilmenite and perovskite is estimated to be P (GPa)=(24.0¿0.5) -(0.0025¿0.0025)(T-800)(¿C) by combining the present results and previous constraints on the slope of the phase boundary from the thermodynamic properties. Measured unit cell parameters of MgSiO3 perovskite indicate that the average volumetric thermal expansion coefficient of MgSiO3 perovskite over 25¿--1200 ¿C is 2.0¿0.4¿10-5 K-1 at 25 GPa. These results provide the phase stability relation of MgSiO3 and the thermal equation of state of MgSiO3 perovskite at pressures corresponding to the boundary between the upper and lower mantle. However, uncertainties involved in extrapolating to the actual mantel temperatures do not allow evaluation of the chemical structure near the 670 km discontinuity in the mantle. ¿ American Geophysical Union 1995.