Abstract: Based on the field observation of the South China Monsoon Rainfall Experiment (SCMREX), the National Centers for Environment Prediction (NCEP) final (FNL) operational global analysis data and the equivalent temperature of the blackbody at the top of the cloud (TBB) data, a comparative analysis of the circulation and related meteorological elements between two severe rainfall events (7-12 and 14-17 May) in South China in 2013 was conducted. The South China Sea (SCS) summer monsoon established in the third pentad of May 2013, just in between two severe rainy stages. There exist significant differences between two events in the major circulation systems, e.g. the South Asia High (SAH) and the subtropical high westerly jet at the higher levels, the circulation pattern at 500 hPa, the water vapor transportation, etc. It indicates typical variances of the effects of the large scale circulation on severe rainfalls before and after the onset of SCS monsoon. During 7-12 May, influenced mainly by both the southward invasion of cold front and the convergence of warm moist airflow from the tropical region, the two rain-bands appeared in northern and southern South China. Differently, due to the vigorous convective activity over South China caused by the strong warm and moist air after the onset of monsoon, the rain-band was formed in the north of Guangdong Province during 14-17 May, with more concentrated and convective precipitation than the former event. The underlying physical mechanisms for the two severe rainfall events are quasi-balanced thermal adjustment process, which is better reflected during the second event because of heavier precipitation. The sounding data in Guangdong are also applied to reveal differences of the atmosphere dynamic and thermodynamic effects between the two warm sector rainstorms. The warm sector rainstorm during 7-12 May occurred under the condition of the atmospheric baroclinic instability induced by the strong low-levels vertical wind shear. However, the strong coupling between the high-level and low-level jet streams caused the warm sector rainstorm during 14-17 May after the onset of the SCS summer monsoon.