Analysis of 1958-1997 NCEP/NCAR pentad dataset from April to June indicates that,the time when the average θ_se|850hPa≥335K and the negative zonal wind changes into positive stably in area(10-20°N,110-120°E) has a good correspondence with the onset date of South China Sea Summer Monsoon and can be used as a norm to judge the latter.Above condition should last at least 3 pentads and the following discontinuous time be less than 2 pentads,or last 2 pentads then has one pentad break then resume at once.