Abstract: Onset and end of the First Rainy Season (FRS) in South China are defined in this paper using daily rainfall data and NCEP/ NCAR reanalysis daily data for the period 1957~ 2004. After choosing a reasonable area and representative stations in South China, evolutions of variables including rainfall, precipitable water (PW), water vapor flux (WVF), vertical velocity, and potential pseudo-equivalent temperature (PPET) are analyzed. Results show that all these variables have experienced obviously staggered changes around the onset and end. The averaged beginning date of FRS occurs in pentad 19 (1st pentad in April), as the precipitation increases outstandingly, in accordance with dramatic increments in PW, WVF and PPET. Meanwhile the WVF changes from divergence to convergence, accompanied by gradually enhanced vertical activities. It rains not too much in April which depicts the features of frontal precipitation. After the East Asian summer monsoon outbreaks in May, South China enters a period with much more precipitation and more vigorous convections. The peak of the rainy season is in June. Not until pentad 34 (the 4th pentad of June) that FRS comes to the end when all these variables happen to experience a sudden decrease. According to the above analyses, a timetable of annual onset and ending dates of FRS in South China is given by choosing threshold criteria. Investigations on the potential height and wind fields before and after the beginning (ending) pentads show that the climatological atmospheric circulations are in favor of the increase in temperature and humidity before the beginning and in favor of the southward movement of cold air after the beggining so as consequently to cause remarkable increase in rainfall. As for the ending of the FRS, it is due to the seasonal transition of the general circulation in East Asia, especially due to the first northward shift of the Western Pacific Subtropical High (WPSH).