Volume measurement of MgSiO3 perovskite was made in the temperature range from 25 to 500 ¿C as a function of pressure up to 20 GPa by in situ x-ray diffraction, using a DIA apparatus combined with synchrotron radiation. The measured thermal expansivity ranges from 1.8 to 2.5 ¿ 10-5 /K and decreases only slightly with pressure. Our present results are in good agreement with the previous lower pressure (up to 11 GPa) data of Wang et al. [1994> and the higher pressure (36 GPa) data of Funamori and Yagi [1993>, but contrast with the results using diamond anvil cell by Mao et al. [1991>, particularly in a low pressure range. ¿ American Geophysical Union 1995