Abstract: Using the OLR(outgoing longwave radiation) data during 1975- 1993 (except for 1978),the characteristics of interseasonal variation of Low frequency oscillation in South China Sea and its relation to the establishment and activity of summer monsoon, are investigated. It is found: (1)The strength of the oscillation is stronger during the summer monsoon period than that during the winter monsoon period. the establishment of summer monsoon is in the negative phase of the first strong oscillation; (2)Low-frequency oscillation closely relates to monsoons activeness and interruption(or weakness), the circulation objects of the oscillation during summer monsoon period are the eastward and westward swing of the subtropical high over west Pacific,and the southward and northward vibration of ITCZ; (3) the intraseasonal oscillation accompanied with seasonal variation makes the summer monsoon experience the evolution from establishment in the first oscillation, the strengthening in the second and the third oscillation and the weakening in the fourth oscillation.