Interannual variation of summer monsoon onset over the South China Sea (SCS) were studied based on the seventeen years (1979~1995) NMC global analysis data and Outgoing Longwave Radiation (OLR) data observed from NOAA polar-orbit satellites. Based on the criteria defind in this paper for the SCS summer monsoon onset, the average onset date from 1979 to 1995 was around the fourth pentad of May. During the SCS monsoon onset, the most direct impact from the vicinity of the SCS is the equatorial westerly winds in the Indian ocean through their eastward extension and northward movement. Meanwhile, there are also indirect impact of the enhancement of the Somali cross-equatorial flow and the vanishing of the Ariabian high at sea, which change the equatorial westerly winds, in the SCS onset. There are quite significant interannual variation in the SCS onset. It is shown that the impact of the equatorial westerly winds in the Indian ocean on the SCS summer monsoon onset and its interannual variation is very important. It has also been found that the interannual variation of the SCS onset are closely related with the ENSO events.