The partition coefficient of Fe and Mg between perovskite and magnesiow¿stite KMg-FePv-Mw has been measured by adopting a B2O3 catalyst technique. The present experimental results demonstrate that KMg-FePv-Mw is a factor of two closer to unity than that obtained in previous studies. KMg-FePv-Mw increases from 0.16 to 0.35 with increasing Mg♯ of magnesiow¿stite from 50 to 86. On the basis of these results, the compositions of enstatite and magnesiow¿stite included in diamond imply that the temperature at the top of the lower mantle is 1870--2040 K. The Mg♯ (100Mg/(Mg+Fe)) of perovskite and magnesiow¿stite at the top of the lower mantle are 93 and 82--83, respectively, for a pyrolitic mantle model. The upper bound of the lower mantle temperature estimated in this study do not allow a larger fraction of perovskite than the pyrolitic mantle. ¿ American Geophysical Union 1996