We report in situ angle-dispersive X-ray diffraction measurements for MgSiO3 at 90--118 and 135--144 GPa and 300--2900 K. At 90--144 GPa, we observed a new diffraction peak at 2.57--2.62 ¿ as reported in Shim et al. <2001>. The pressure-induced position shift of the new peak is consistent with those of MgSiO3 perovskite (Mg-Pv), supporting that the new peak is associated with a modification in crystal structure of Mg-Pv at 88 GPa. During laser heating above 2500 K at 144 ¿ 10 GPa, in addition to peaks of Mg-Pv, we observed new peaks as well as peak splittings and intensity changes of some peaks. The major features in the diffraction patterns can be explained by the modified Mg-Pv proposed by Shim et al. <2001> together with the post-perovskite phase proposed by Murakami et al. <2004>. This supports the existence of a major phase transition in MgSiO3 under conditions close to those at the base of Earth's mantle.