Using 1975-1993 (with 1978 missing) data of the outgoing longwave radiation (OLR), characteristics of seasonal variation of low-frequency oscillations in the South China Sea and its relation to the establishment and activity of the summer monsoon there are studied. As is shown in the result, the low-frequency oscillation in the South China Sea is much stronger in the period of summer monsoon than in that of winter monsoon and the summer monsoon there usually begins to set up in a negative phase of the first significant low-frequency oscillation for the early summer. The study also reveals that the circulation for the low-frequency oscillation during the summer monsoon in the Sea is embodied as north-south fluctuations of the ITCZ and east-west shifts of western ridge point of the West Pacific subtropical high, suggesting close correlation between the low-frequency oscillation and the active and break (decay) of the South China Sea monsoon. In the meantime. the work illustrates how the low-frequency oscillation in the South China Sea are superimposed with the seasonal variation of the general circulation. so that the summer inonsoon covers the establishment of the I st, intensification of the 2nd and 3rd the low-frequency oscillations and decay of the 4th oscillation. Using 1975-1993 with 1978 missing data of the outgoing longwave radiation (OLR), characteristics of seasonal variation of low-frequency oscillations in the South China Sea and its relation to the establishment and activity of the summer monsoon there are studied. As is shown in the result, the low-frequency oscillation in the South China Sea is much stronger in the period of summer monsoon than in that of winter monsoon and the summer monsoon there usually begins to set up in a negative phase of the first significant low -frequency oscillation for the early summer. The study also reveals that the the circulation for the low-frequency oscillation during the summer monsoon in the Sea isembodied as north-south fluctuations of the ITCZ ​​and east-west shifts of western ridge point of the West Pacific subtropical high, suggesting close correlation between the low-frequency oscillation and the active and break (decay) of the South China Sea monsoon. In the meantime. The work illustrate how th e that the summer inonsoon covers the establishment of the I st, intensification of the 2nd and 3rd the low-frequency oscillations and decay of the 4th oscillation.