Partial melting experiments were carried on KLB-1 peridotite, a xenolith sample from the Earth's upper mantle, at 1.5 GPa and temperatures from 1 300 to 1 600 degrees C, with heating time varies from 1 to 30 min. We quantify the axial temperature gradient in the deformation-DIA apparatus (D-DIA) and constrain the time scale of partial melting by comparing experimental observations with calculated result from pMELTS program. The compositions of the liquid phase and the coexisting solid phases (clinopyroxene, orthopyroxene, and olivine) agree well with those calculated from pMELTS program, suggesting that local chemical equilibrium achieves during partial melting, although longer heating time is required to homogenize the bulk sample. The Mg-# (=Mg/(Mg+Fe) mol.%) of olivines from the 1-minute heating experiment changed continuously along the axial of the graphite capsule. A thermal gradient of 50 degrees C/mm was calculated by comparing the Mg-# of olivine grains with the output of pMELTS program. Olivine grains at the hot end of the graphite capsule from the three experiments heated at 1 400 degrees C but with different annealing time show consistence on Mg-#, indicating that partitioning of Fe2+ between the olivine grains and the silicate melt happened fast, and partial melting occurs in seconds.