Abstract: In the context of NCEP/NCAR reanalysis daily data in 1998, the features of summer monsoon LFO (Low-Frequency Oscillation) and LFR (Low-Frequency Rainfall) over the Beijing-Tianjing-Hebie area in 1998 are studied. Results show that there is a 15-35 d oscillation cycle in summer monsoon in the target region. The LFR is weaker in May and June than in July and August. The migration direction of LFR is more likely the same as the low frequency (LF) divergence at the 200 hPa level. Vapor over the target region mainly comes from the south and west sides of the research area. The variation of LF vapor transportation from south wind can be useful for forecasting the occurence and development of LFR.