The South China Sea Monsoon Experiment (SCSMEX) was conducted in South China Sea (SCS) and surrounding areas during MayæJune 1998. As the first comprehensive field experiment aiming on the South East Asia summer monsoon, a main purpose of the mesoscale program is to define the organization and dynamics of precipitation processes associated with the onset and mature phase of the SCS monsoon. In the early May, the SCS region was mainly under the control of the subtropical high over the western Pacific. On the western flank of the subtropical high, low-level airflow exhibited southeasterly winds in the southern SCS and southwesterly winds in the northern SCS. The low-level divergent field prevented deep convection from developing, although isolated cumulonimbus cells and scattered precipitating congestus were frequently observed. The monsoon onset started at low-level from the northern SCS on 15 May and progressed to the southern SCS on 20 May. The monsoon onset at the upper-level followed and completed by 24 May. During this transient period, there were two types of convection over the SCS region. The first type was the well-organized cloud system related to the frontal systems from northwestern China. The convection had a large areal coverage with wide spread rainfall. The second type of convention was mainly related to the mesoscale vortex developed in northern Indochina peninsula and southern China. The associated rainfall was relatively localized but with great intensity. The dry days may also occur during the onset period when no systematic influence from either north or west. A couple of case studies is performed to investigate the rainfall characteristics and kinematic structure of monsoon convection. The case of 15 May, an example of the first type of convection and case of 24 May, a representative of the second type of convection during the onset period, exhibits significant departures from the archetypal tropical oceanic MCS observed in other geographical regions. The differences include: 1) most of the mesoscale convection forming in the SCS region has much less stratiform rainfall compared to typical oceanic MCS, 2) rather than the common trailing mode of stratiform rain, the limited stratiform clouds of mesoscale convection during the onset period have a leading mode to the convective counterpart, and 3) instead of having a narrow ribbon of maximum low-level convergence and updraft at the leading edge of the system, the convection observed during the onset period shows a wide band of convergence and updraft in the far rear portion of the system. Consequently, the new cells form behind, not ahead of, the old cell. The new cell formation in the rear part implies that the low-level inflow has to pass a convective precipitating area. Therefore, the inflow could be chilled by the cool air caused by evaporative cooling. This may reduce the strength of updrafts and have negative effects on the development of the convection. To maintain such an atypical kinematic structure, one of the following conditions should be met: a) there is a strong lifting along the edge of the cold surface outflow, e.g., on 15 May, or b) the thermodynamic stratification is near neutral, e.g., on 24 May.