Based on the tropical cyclone data from the China Meteorological Administration and the NCEP/NCAR reanalysis data, the characteristics of the Intraseasonal Oscillation (ISO) of the intensity of convection over Intertropical Convergence Zone (ITCZ) and its influence on the periodical tropical cyclogenesis over the western North Pacific are studied. In addition, the relationships between the ISO of ITCZ and equatorial westerly, cross-equatorial flow averaged from 105°E to 110°E, and the easterly to the north of ITCZ are investigated. The results show that: (1) The significant signal of 30－60-d oscillation exists in the intensity of convection over ITCZ. The active and inactive convections over ITCZ occur in turn at the period of 30－60 d. About two-thirds of tropical cyclones over the western North Pacific (5°N－20°N, 120°E－150°E) occur in the active phase of ISO. (2) It is quite clear that the 30－60-d oscillation of ITCZ propagates eastwards slowly in the equatorial region, but westwards in the subtropical regions over 25°N－35°N. And it is also noticed that there are obvious differences of ISO between the strong ISO years and the weak ISO years. In the strong ISO years, the oscillation spreads from the equator to 15°N, and merges here with the oscillation from north to enhance the convection over ITCZ. Therefore more tropical cyclones originate during this period. On the contrary, in the weak ISO years, the oscillation directly propagates northwards to the vicinity of 30°N, results in the inactive convection over ITCZ near 15°N. At the same time, the frequency of tropical cyclogenesis decreases. (3) There are also remarkable 30－60-d oscillation features for the equatorial westerly, the equatorial flow averaged from 105°E to 110°E, and the easterly to the north of ITCZ. Generally, the 30－60-d oscillations of the three flows are consistent with the ISO of ITCZ. However, the relationship between the easterly wind and the ISO of ITCZ is slightly weaker than the other two flows.