We have determined the post-perovskite phase transition boundary in MgSiO3 by in-situ X-ray diffraction measurements, using Au and MgO as internal pressure standards. Results demonstrate that phase transition occurs at 113 GPa and 2400 K with a positive Clapeyron slope of +4.7 MPa/K, based on the Au pressure scale. On the other hand, the simultaneous measurements of MgO standard show that the phase boundary is located at 119 GPa and 2400 K with a Clapeyron slope of +11.5 MPa/K, consistently with the previous experimental results based on the Pt scale and the theoretical calculations. Our experiments also indicate that the stabilities of perovskite and post-perovskite in all of (Mg0.89Fe0.11)2SiO4, natural pyrolitic mantle, and MORB compositions are consistent with the phase transition boundary in pure MgSiO3, when all the data are compared using the same Au scale. The compositional variations in natural systems have little effect on the post-perovskite phase transition.