Abstract: The summer monsoon onset over the northern South China Sea (SCS) in 1998 (May 16 to 20) was characterized by abrupt onset of meso-scale convective activities a nd rapid increase of precipitation. The possible mechanism for formation of the meso-scale rain belts was revealed through discussing the physical conditions o f occurrence of meso-scale convective system and precipitation under the large -scale background: (1) The high pseudo equivalent potential temperature and convective instability in the lower troposphere, low-level southwesterly confluence and high-level divergence over South China and the northern SCS provided fav orable large-scale thermodynamic and dynamic conditions for development of the meso-scale convective systems. The southwest flow from the bay of bengal interacted with that from the subtropical high, which constituted the major moisture c hannel, thus bringing about deep wet layers and strong moisture confluence; (2)Trigged by two cold troughs from high and mid latitudes, the convective activit ies over the northern SCS broke out abruptly; (3)Analysis of retrieved precipita tion based on the dual-Doppler radar during South China Sea Monsoon Experiment (SCSMEX) indicated that active convection influenced by the monsoon trough and c orresponding wind shear line organized and formed continually some meso-scale convective rain belts. During May 15 to 19, about twelve precipitation processes with 6～12 hours life span or more were observed; (4) Under the favorable synop tic conditions, establishment of the monsoon trough and shear line in the low level, as well as occurrence and development of meso-scale low vortex were all n ecessary conditions for the continuous formation and maintenance of mesoscale convective systems. 