Phase relations in Mg3Al2Si3O12 (MgSiO3 + 25 mol% Al2O3; pyrope garnet composition) were investigated on the basis of in-situ synchrotron X-ray diffraction measurements at high-pressure and -temperature in a laser-heated diamond anvil cell (LHDAC). Results demonstrate that perovskite is solely stable up to 140 GPa and 2200 K, and perovskite and CaIrO3-type post-perovskite phase (space group: Cmcm) coexist above 140 GPa and 2200 K. Post-perovskite is formed as a single phase above 150--170 GPa. Previous study has shown that pure MgSiO3 perovskite transforms to post-perovskite phase above 125 GPa and 2500 K based on the same pressure standard. These results indicate that addition of Al2O3 expands the stability of MgSiO3 perovskite relative to post-perovskite.