Based on the tropical cyclone best track data from the Joint Typhoon Warning Center (JTWC) and the NCEP/NCAR reanalysis data from 1980 to 2009,we set a threshold for rapid intensification tropical cyclones (RITC) as 35 kn,which was the magnitude of intensity change of tropical cyclones in a period of 24 h with the accumulative probability of 96% over the western North Pacific.According to zonal wind shear between 200 hPa and 850 hPa,the paper divided the vertical wind shear (WS) into the westerly vertical wind shear (WWS) and easterly vertical wind shear (EWS) and compared the characteristics of rapid intensification of tropical cyclones in EWSwith those inWWS.The results show that about 70% of RITCs occur inEWS.There are most RITCs in September,whose initial intensity are between 65 and 70 kn.In largeEWS,abundant vapor comes from southwest at 850 hPa.At 500 hPa and 200 hPa,the Pacific subtropical anticyclone is strong and the ridge line is situated in north area.The temperature in outflow layer is below -79 ℃.In large WWS,the water vapor is from southeast at 850 hPa.At 500 hPa,subtropical high is split into several centers.Compared with that of the former,the divergence field of RITCs at 200 hPa is weaker.And the temperature in outflow layer is about -76 ℃.