Classification studies were conducted in the paper on the temporal and spatial variations as well as the effect of water vapor on the tropical cyclone(TC) frequency over the South China Sea(SCS) and Northwest Pacific Ocean(WNP) in different periods during 1961-2010.The major results are summarized as follows:The whole year can be divided into an active period(June-November) and a calm period(December-May) for TC.The first eigenvectors during both periods show a uniform enhancement or reduction of the TC frequency over the SCS and WNP,while the second eigenvector in the active period shows that the TC frequency over the WNP is in opposite phase with that over the SCS,and the second eigenvector in the calm period shows that the TC frequency to the west of 130°E is in opposite phase with that between 130°E and 150°E.Correlation and composite analysis indicates that the divergence of water vapor flux shows a significant negative correlation with the TC frequency over the WNP in both periods and over the eastern SCS and the vicinity of Philippines in the calm period,but they show only a weak negative correlation over the central and northern SCS in the active period.Therefore,the water vapor conditions may lead to higher TC frequency over the northern(eastern) SCS and the central and eastern(western) parts of the WNP during the active(calm) period.