This is the first report of the direct measurement of &agr;-&bgr; transformation kinetics in Mg2SiO4, a major constituent in the Earth's mantle, through in situ X ray diffraction experiments at high pressure. The experiments were conducted using the sintered diamond cubic-anvil apparatus with synchrotron radiation (MAX 80). Although the present work is still preliminary, the kinetic data were analyzed using a nucleation and growth model. The growth rate of &bgr;-phase is determined to be 3.3 (1.6~4.9)¿10-10 m/s at 13.0 GPa and 900 ¿C, 7.1 (3.4~10)¿10-10 m/s at 14.1 GPa and 885 ¿C, and 1.5 (0.7~2.3)¿10-9 m/s at 14.9 GPa and 875 ¿C. ¿ 1998 American Geophysical Union