The tropical low frequency oscillation and its possible influence on the South China Sea (SCS) summer monsoon are studied by using the daily NCEP/NCAR-2 (National Centers for Environmental Prediction and National Center for Atmospheric Research) reanalysis data and the outgoing long-wave radiation (OLR) data from NOAA (National Oceanic and Atmospheric Administration) for the time period of 1979－2008, and the CPC Merged Analysis of Precipitation (CMAP) data for the time period of 1979－2007. Results reveal that there are significant low frequency oscillations with 30－60 day period over the SCS region. When the 30－60 day oscillation is in an active state, the low frequency components of southwesterly prevail over the SCS and the cyclonic circulation appears in northern SCS. Hence, in the active state the low frequency oscillation tends to strengthen the South China Sea summer monsoon (SCSSM) trough and the SCS summer monsoon. In the inactive state of the low frequency oscillation, the situation tends to be opposite. Further researches indicate that the spatial pattern for the variability of the low frequency oscillation is analogous to that for the interannual variations of the SCS summer monsoon. Particularly the variance of low frequency oscillation activity in the SCS region can account for nearly half of that for the interannual variations of the SCS summer monsoon. The comparison between strong and weak SCS summer monsoon years presents that the occurrence probability of active state is higher than that of inactive state for the low frequency oscillation in strong monsoon years. However, in weak monsoon years the occurrence probability of inactive state is higher than that of active state. Therefore, the low frequency oscillation with 30－60 day period likely has an important influence on the SCS summer monsoon. When the active state is dominant for the low frequency oscillation, the SCS summer monsoon tends to be strong. On the contrary, the SCS summer monsoon tends to be weak when the inactive state is dominant for the low frequency oscillation.