Based on pentad and monthly mean wind and specific humidity data of NCEP from 1958 to 1997 and precipitation data of 32 stations in Yunnan,the activities of the Bay of Bengal(BB) monsoon and the correlation relation between BB monsoon and precipitation in early summer of Yunnan have been studied,and we got some interesting evidences.First,on 850 hPa,comparing with SCS and Arab zonal wind,the appearing of BB zonal wind is the earliest and the its variation is similar to that of SCS zonal wind during summer monsoon.Second,the onset date of southern BB summer monsoon is 6 pentads in April and that of northern BB summer monsoon is 3 pentads in May,and it is the earliest in the onset of Asian summer monsoon.The onset of southern BB summer monsoon is earlier than onset of northern BB summer monsoon.Last,May precipitation in Yunnan increases while the onset of BB summer monsoon islaterabout 2~3 pentads,whereas more and less May precipitation in Yunnan is closely relative to the activities of cold air at mid-latitude,it may increases or decreases while the onset of BB summer monsoon is early or later about 1 pentad.