The SST variation characteristics over the South China Sea (SCS) and the Bay of Bengal before and after the onset of summer monsoon and the possible relation with Southwest air current are investigated by means of the NCEP/NCAR weeklymean SST reanalysis data and dailymean surface wind data from 1982 to 1999, using synthetic and correlation analysis technique. The results show that the SST increase on SCS and the Bay of Bengal is not only related to ocean phenomena, but also to atmospheric factors in the two weeks before the monsoon onset. There were great difference in the regional SST variability between SCS and Bay of Bengal in the two weeks after monsoon onset. The SST increased in the east coast and the northwest part of the SCS and the Bay of Bengal, and decreased in the southwest and central parts of the SCS and the Bay of Bengal. The disparity distributions of SST variation are directly related with the surface southwest current variation in the two weeks after monsoon onset.