The mixed layer heat balance in the western North Pacific is examined using historical temperature data and U.S. National Centers for Environmental Prediction reanalysis surface wind and heat flux. Although the data come from different sources, the mixed layer heat budget is balanced reasonably well in the region studied. For the annual average, the surface thermal forcing is significant mainly in three regions. One is in the Mindanao Dome, where the incoming surface heat flux is balanced in a large part by the vertical entrainment induced by Ekman pumping. The second occurs in the Kuroshio region, where much of the heat advected by the Kuroshio is released to the atmosphere. The third is located in the central subtropical gyre where the annually integrated surface thermal forcing is balanced primarily by vertical entrainment associated with the deepening of the mixed layer. Although seasonal variation in sea surface temperature (SST) is mainly due to surface thermal forcing, ocean dynamics also has an effect, particularly, during the developing stage of the summer monsoon. From late May to early July, there is a northeastward increase in SST (>29¿C) in the region 120¿--160¿E, 10¿--20¿N, which coincides with the northeastward onset of the summer monsoon. As the summer monsoon develops, vertical entrainment becomes effective at cooling, leading to a decrease in SST against the incoming surface heat flux over a large part of the western North Pacific.