The phase boundary of the post-spinel transition in Mg 2 SiO 4 was re-investigated by means of high P-T in situ X-ray diffractometry with a gold pressure marker in a Kawai-type apparatus. Rapid and continuous temperature changes were conducted to initiate dissociation of spinel, which tends to be inert after long annealing. Isothermal decompression at high temperature was conducted to form spinel from perovskite plus periclase. The phase boundary is located at ca. 22GPa in the temperature range from 1550 to 2100K, which is 1-1.5GPa lower than the 660km discontinuity. This discrepancy might be explained in terms of the pressure effect of thermocouple emf and inaccurate equation of state (EOS) for the pressure maker. The transition is found to be less sensitive to temperature than reported previously, with a Clapeyron slope ranging from -2 to -0.4MPa/K. This small Clapeyron slope implies that the post-spinel transition would not be an effective barrier to mantle convection.