The stability of Mg2SiO4, a major constituent in the Earth’s mantle, has been investigated experimentally by in situ observation with synchrotron radiation. A cubic-type high-pressure apparatus equipped with sintered diamond anvils has been used over pressures of 11 to 15 gigapascals and temperatures of 800 degrees to 1600 degrees C. The phase stability of alpha-Mg2SiO4 and beta-Mg2SiO4 was determined by taking account of the kinetic behavior of transition. The phase boundary between alpha-Mg2SiO4 and beta-Mg2SiO4 is approximated by the linear expression P = (9.3 +/- 0.1) + (0.0036 + 0.0002)T, where P is pressure in gigapascals and T is temperature in degrees Celsius.