The tropical cyclone frequencies in the northwestern Pacific in 2010 and 1998 are far less than the normal frequency,so the diagnostic analyses are performed on relationships between ENSO events,East Asian summer monsoon circulation anomalies and the tropical cyclone frequency anomalies based on the data of serially numbered typhoons in the northwestern Pacific,the data derived from NCEP/NCAR reanalysis dataset and the interpolated outgoing longwave radiation(OLR) data provided by NOAA.The synoptic images of the main memberships of East Asian summer monsoon system influencing the tropical cyclone frequency are given:From spring to summer,the sea surface temperature in the equatorial eastern Pacific is warmer,Hadley circulation is stronger,and Walker circulation is weaker;the West Pacific subtropical high is intensified and moves westward;the monsoon trough extends southward without eastward and westward spread;the cross-equatorial flow in South China Sea and the western Pacific is weaker;the abnormal apparent heat source and apparent moist sink extends southward,and the tropical convective activity is not active in South China Sea and the Philippines,but it is active in south of the equator;therefore the tropical cyclone frequency is significantly less than the normal and the tropical cyclone moves westward.