Abstract: The characteristics and interannual variation of Intraseasonal Oscillation (ISO) of the South China Sea (SCS) summer monsoon are preliminarily examined by Butterworth band-pass filtering method, using the NCEP/ NCAR reanalysis wind fields and vertical speed and the NOAA outgoing longwave radiation (OLR) data. The authors first project 850-hPa wind to the southwest (Vsw), then process it with 30－60 days band-pass filtering method, calculate its absolute value in the SCS area, and then calculate the average value during the summer time. After that, the authors use it to measure the activity intensity of the SCS summer monsoon ISO (SCSSISO).The authors calculate the average value of SCSSISO in the SCS area, and then, define it as the SCSISO index. Climatologically, there exist five ISO activity centers over the Asian summer monsoon region, including the SCS (5°N－15°N, 110°E－120°E). The authors select the above-mentioned area, calculate the summer average value in the area, then get the day-to-day sequence of climate-average 850-hPa Vsw after 30－60 days band-pass filtering in the SCS region. There are also three obvious active processes of the SCS ISO from May to September, under average climate condition. The authors standardize the time series of the SCSSISO index from 1979 to 2007, recognise years with the index more than 1 as active, and less than -1 inactive. The active years are 1979, 1996, 1999, 2007 and the inactive years are 1993, 1994, 1995, 1998 from 1979 to 2007. During the active years, the SCSSISO starts to become markedly stronger from early summer (early May). However, the SCSSISO is very weak throughout all the year, during inactive years. In order to study the circulation features of ISO activity in the SCS summer monsoon region, relatively complete and significant fluctuations of the daily ISO activity in the SCS summer monsoon are divided into eight phases. Phase 1 to phase 4 are positive phases and phase 5 to 8 negative. The authors find that, in the eight phases of a complete cycle activity in the SCSSISO active years, the low-frequency convection center first starts moving eastward from the tropical western Indian Ocean, then turns northeastward, and finally arrive in the northwestern Pacific Ocean. New ISO forms in the west of the Indian Ocean in phase 8 and a new cycle begins there.The factors are also studied, which have impacts on the interannual variability of the ISO activity in the SCS summer monsoon. Seasonal average of the 850-hPa winds and OLR in the prior spring (MAM), the difference between upper-level and low-level (200 hPa-850 hPa) zonal winds averaged in spring, and the profile of spring averaged vertical velocity along the equator are calculated using the difference of the composite OLR and winds between the active and inactive years, to find out the relationship and differences between the prior convections and the tropical zonal circulations. The results reveal that the region near the SCS has a low OLR center and stronger Walker circulation in the prior spring of the SCSSISO active years. The stronger Walker circulation in spring which affects the northward propagation of ITCZ and then forms a strong center of low-frequency convection near the SCS in spring, can be one of the signals of the interannual variability of ISO activity.