The South China Sea Monsoon Experiment (SCSMEX) was conducted in South China Sea (SCS) and surrounding areas during MayæJune 1998. The primary goal of the experiment is to provide a better understanding of the key physical processes for the onset, maintenance and variability of the monsoon over the Southeast Asia and southern China leading to improved monsoon predictions. As the first comprehensive field experiment aiming on the SCS summer monsoon, the goals of the mesoscale program associated with the primary objective are: 1) to define the initiation, structure, evolution and dynamics of precipitation processes associated with the onset and mature phase of the SCS monsoon, and 2) to obtain quantitative rainfall estimates, vertical air motion and inferences on microphysical structure over a tropical oceanic site. Through a review of precipitating systems during the first IOP, we found that there were two types of convection over the South China Sea during monsoon onset. The first type was the well-organized cloud system originally from the frontal systems from northwestern China. The convection had a large areal coverage with wide spread rainfall. On its way to the South China Sea, the convection may get reorganized with different orientation and propagation. An enhancement of the intensity of the system may also occur. The second type of convention mainly erupted in the northern South China Sea with less help of synoptic-scale system. They usually consisted of several narrow rainbands, either separated or forming a long squall line type of system. The rainfall was relatively localized without significant amount. In this study, we will investigate the organization and evolution of deep mesoscale convective systems observed by Doppler radar network on May 15 during the early onset of the South China Sea summer monsoon. The system studied here is a good representative of the first type of convection during monsoon onset. The detailed rainfall structure and three-dimensional kinematic fields of the system are examined by dual-Doppler radar synthesis and will be presented at the conference.