Abstract: The monsoon region of western North Pacific（WNP）(5°－25°N，120°－160°E) is a n important part of Asian-Pacific and AsianAustralian monsoon region, and is a distinct monsoon region. The climate features of WNP summer monsoon have been studied. The results show that WNP summer monsoon onsets in pentad 31 （early June） or so. After onset, summer monsoon experiences three active-break cycles. Every cycle has a dry and wet period. The first cycle is from early June to mid -July and the center of precipitation and lowerlevel west wind is mainly loca ted between 5°-10°N. The second cycle is from late July to late September, pre cipitation and lower-level west wind move to northeast obviously and are locate d between 10°-20°N. In this cycle, WNP summer monsoon is the most prosperous. The third one is from late September to early October and is the weakest period, in which the precipitation and lower-level west wind retreat south again and lie between 5°-10°N. The end of the third cycle indicates the end of WNP summ er monsoon. There are obviously intraseasonal oscillation （ISO） in WNP monsoon region, this climatological ISO is mainly composed of 30-60 and 10-20 day oscil lation. But low-frequency oscillation of 30-60 days is significant. Base on the analysis of convection and lower-level west wind distributing in different phases over WNP, low-frequency convection and west wind propagate to northwest. To a large extent, the active-break cycle of monsoon rain, convection and lower-level west wind over WNP are all modulated by 30-60 and 10-20 day low-frequency oscillation.