Abstract: The apparent heat sources, apparent moisture sinks and large-scale wind, temper ature as well as surface pressure during the summer monsoon onset over the north ern South China Sea (SCS) in 1998 were investigated. The results suggested that there was a kind of positive feedback mechanism between large-scale circulation s and meso-scale convective systems (MCSs ). At the early stage of the monsoon onset, the large-scale background provided favorable synoptic and dynamic cond itions for the summer monsoon onset and formation of mesoscale convective acti vities; whereas after the summer monsoon onset, occurrence of persistent and large-scale meso-scale convective activities produced obvious feedback effect on largescale circulations. Because of the release of latent heating driven by e nhanced convective activities, the intense atmospheric heating appeared over the northern SCS, which resulted in: (1) the meridional temperature gradient over the SCS reversed from upper-level to low-level and then the large-scale circul ations were changed seasonally;(2) correspondingly, the surface pressure over the northern SCS deepened continually and formed broad monsoon trough and obvious pressurereducing areas, thus making the subtropical high moving out of the SCS eventually; (3) with the development of the low-pressure circulations in the mi ddle and low troposphere, the MCSs further enhanced and extended southward,which was favorable to the SCS monsoon onset and maintaining over the middle and sout hern SCS; (4) the deepening of monsoon trough promoted strengthening of the monsoon flow and moisture transport on the southern side of it, consequently the monsoon onset reaching prevalence.