In this paper we investigated the variations of summer montoon in the South China Sea.We found that there are two kinds of oscillations in the intensity of the monsoon, one has a period of forty days, the other, fourteen days.We also found that the oscillation of a period of forty days is caused by the alternative extension of the monsoon trough eastward in the South China Sea and the extension of the subtropical anticyclone westward in the West Pacific. It is our opinion that the summer monsoon in the West Pacific and the South China Sea is a system which is independent of the Indian monsoon and that the dividing line between these two monsoon systems is at the longitude 100°E.