Abstract: Climatological characteristics of tropical cyclones in the Northwestern Pacific during 1951—2006 areinvestigated based on the tropical cyclone best track data from China Meteorological Administration.Tropical cyclones are classified into 6 different categories to study the long-term variation under the global warming background.From 1951 to 2006, nearly 33.9 tropical cyclones occur in the Northwestern Pacific every year.Tropical depressions contribute 18.8% and super typhoons contribute 17.8%.The total frequency of tropical cyclones in the Northwestern Pacific decreases since 1950s and the decreasing trend is1.8 times every 10 years.Frequency of tropical cyclones is highest in 1967 and lo west in 1998.The decrease trend of tropical cyclones frequency is mainly caused by tropical depression and super typhoon Ⅱ(maximum wind≥58 m · s ^-1, super typhoon Ⅱ for simple).The decrease trend of super typhoon Ⅱ ismost remarkable.Contributions of tropical depression and super typhoon Ⅱ frequencies decrease whilecontributions of other categories increase.Characteristics of frequency, intensity, the first and last date of super typhoon Ⅱ are different fromthose of other categories.About 56.3%of tropical cyclones in the Northwestern Pacific appear in summerseason, especially in August.The average monthly frequency is 7.9 in August and 0.3 in February.Withthe growing intensity of tropical cyclone, the maximum frequency postulate from August to October.Themaxim um frequency of super typhoon appears in autumn while those of other categories appear in August.Monthly frequencies of tropical depression and super typhoon Ⅱ take on decrease trends in autumn whilethose of other categories don' t show remarkable trends.The annual variations of super typhoon Ⅱ arenegative related with the annual variations of other categories except super typhoon Ⅰ.Yearly mean maxim um speed and minimum depression of tropical cyclones show a decrease trend.Yearly maximum speed oftropical cyclones decreases about 6.5 m · s ^-1 every 10 years, while yearly minimum depression has increased especially since 1987.Decrease trend of maximum speed is mainly caused by the decrease frequencyof super typhoon Ⅱ.The average first date of tropical cyclones is around 20 February and the average lastdate of tropical cyclones is around 15 December.The first date of tropical cyclones shows a slight delayingtrend.The first date of super typhoon Ⅱ have remarkable long-term delaying trend of 25 day s every 10y ears while the last date displays long-term advancing trend of 9 day s every 10 years.The last date of tropical storm has delaying trend while that of super typhoon Ⅰ has advancing trend.