Abstract: Employing OLR and TBB data,we presented an index and decided the date of summer monsoon onset over South China Sea(SCS) during the period from 1975-1993, which is consistent with the date ofrecurvature of wind.On the basis of this,we discussed the pro cesses of summer monsoon over SCS.Furthermore,the relationship between summer monsoo nonsetover SCS and sea temperature anomaly,thermal regime over Tibetan Plateau as wellas variation of temperature difference between land and sea was discussed.we found it relates to SSTA in SCS,equatorial east Pacific and region between 30-40°N in Apriland it relates to changing of temperature difference land and sea from status of cool land and warm sea into status of warm land and cool sea in winter.