The South China Sea (SCS) is among areas in the Northwest Pacific most frequented by tropical cyclones (TCs) with intensity reaching a tropical storm or stronger. It is also an area of significant TC genesis. In this study, TC genesis in SCS and its monsoonal variability for 1948–2003 are analyzed. Altogether, in May–September (southwest monsoon period) 157 TC geneses have occurred north of 12°N in SCS, while in October–December (northeast monsoon period) 64 out of 65 TC geneses have happened south of 18°N. It is found that the monsoonal characteristics of the SCS basically determine the region of TC genesis in each monsoon season. Winter TC genesis in the SCS happens over the region where the marine environment satisfies the four criterions on, respectively, the sea surface temperature (SST), mid-troposphere relative humidity, vertical shear of the horizontal winds and low-level atmospheric vorticity. During the summer, as the two criterions on SST and the mid-troposphere relative humidity are satisfied for the whole SCS, TC genesis occurs in the region where both the low-level vorticity and the vertical shear satisfy the criterion. In addition, there is likely more TC genesis in the winter during the onset of La Nina, and more TC genesis in the summer following the onset of El Nino.