Abstract: The characteristics of summer monsoon onset in the South China Sea (SCS) were investigated based on 17 years (1979-1995) NMC wind data and OLR data from NOAA. It is proved that the onset of summer monsoon occurs later in SCS region than in Indo-China peninsula and island areas of Philippines. But the summer monsoon in the eastern part of SCS onsets almost at the same time as that in the western part of SCS. This simultaneity has somewhat standing wave feature. It is also confirmed that in most years there is a equatorial vortex forming near 105°E in April or May and the vortex leads the equatorial westerly in its upper reaches and the westerly in the Southern Hemi-sphere to the southern part of SCS. This creats a favorable condition for onset of the SCS summer monsoon. In the years when the equatorial vortex is unactive, the summer monsoon in the SCS onsets late. There is some relationship between the equatorial vortex and the onset date of SCS summer monsoon. Inearly April, the forming of the vortex and the preliminary building of cross-equatorial flows at 105°E is simult aneous. In May, these cross-equatorial flows enhance gradually and the activity of SCS summer monsoon may be closely associated with these flows. If the 105°E equatorial vortex is called onset vortex of East Asia, it is sure that this vortex is different from that of South Asia. In South Asia the onset vortex relates with monsoon onset directly, but in East Asia it does indirectly. So this also shows that East Asia Monsoon is more complicated than Indian Monsoon.