Abstract: The evolutions of the troposphere temperature fields over Indian and South China Sea (SCS) monsoon areas and their thermal mechani sms are comparatively analysed during the period from March to June, 1996. The results show that the onsets of Indian summer monsoon and SCS summer monsoon are closely associated with the seasonal warming in the troposphere over the zonal belt of 10-30°N in the two monsoon areas above, which leads to the inversion of meridional temperature gradient. During the pre-onset period, the warming over the SCS monsoon region is mainly due to the warm horizontal advection and diabatic heating(latent heat) processes, meanwhile the warming is suppressed by the vertical adiabatic process (cooling). In spring over Indian monsoon region, the significant adiabatic heating due to the subidence motion, which compensates the cooling due to the strong cold advection and diabatic cooling processes, results in a larger warming rate than over the SCS monsoon region. However, the meridional temperature gradient over Indian monsoon region is so large during the late winter and early spring that it takes longer time to warm the troposphere to have the reversion of meridional temperature gradient than it does over the SCS monsoon region, which leads to the phenomenon that the SCS summer monsoon outbreaks earlier than the Indian summer monsoon in general.