The evolution and structure of a mesoscale convective system (MCS) in the South China Sea (SCS) region are documented for the first time mainly using the dual-Doppler radar dataset collected during the South China Sea Monsoon Experiment (SCSMEX) in 1998. In particular, this study focuses on the convection associated with the subtropical frontal passage during the early onset of South East Asian Monsoon (SEAM). The onset of SEAM in 1998 started on 15 May when the two branches of southwesterly tropical airflow arrived in the northern SCS. At the same time, a southeastward moving frontal system from mainland China moved to the monsoon onset area. Different from the frontal rainband prior to the SEAM onset that dissipated rapidly after passing the coastal region, this frontal system was able to produce heavy rainfall in the SCSMEX radar observational domain with significant local intensification. The interaction between the tropical monsoon flow and the frontal circulation played an important role to the organization and structure of the mesoscale convection. In the pre-frontal region, the southwesterly monsoon flow converged with the original southwesterly frontal flow to generate a northeast-to-southwest oriented convective line. In the post-frontal region, the southwesterly monsoon flow converged with the northerly frontal flow to produce a wide convective line with an east-to-west orientation. In addition, the convergence between the southerly monsoon flow and the northerly post-frontal flow generated a deeper and stronger low-level convergence. Thus, the post-frontal convection was more intense and taller than the pre-frontal convection. The precipitation and kinematic structure of the MCS are studied with special attention on their significant departures from the archetypal tropical oceanic MCS. On 15 May, both pre- and post-frontal convection showed a straight upward rainfall and updraft pattern with little tilt as a result of moderate vertical wind shear. In the pre-frontal region, the maximum low-level convergence and updraft of the pre-frontal convection was 20-30 km behind instead of within 1-2 km to the leading edge. Although the convection was intense with maximum reflectivity over 50 dBZ, the stratiform region was very limited as a result of a dry environmental upper layer. The observed MCS had a tendency to form stratiform rain ahead of the convective rain, and two different modes of the leading stratiform structure were found separately in pre- and post-frontal convection.