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The NCEP/NCAR and JTWC (Joint Typhoon Warning Center) reanalysis data are used to investigate the trends of the frequency, distribution and oscillation periods of maximum wind speed and environmentalwind vertical shear of tropical cyclones over the Northwestern Pacific from May to October, 1974 to 2004. Results show that there are contrary variation trends between maximum wind speed and environmentalwind vertical shear. The weak vertical shear is propitious to the development of tropical cyclones. The impact of the vertical shear on intensity variation of tropical cyclones at 12 hours before is the greatest. In strong TC years, the environmentalwind vertical shear is relatively small, and there are more tropical cyclones; while in weak TC years, the environmentalwind vertical shear is large, and there are less tropical cyclones.