Investigating the variability of tropical cyclone (TC) activity in the western North Pacific (WNP) during the 49-yr period 1951-99, interdecadal variation is confirmed. Two kinds of periods are identified in this study: High Frequency Period (HFP) when TC activity is enhanced, and Low Frequency Period (LFP) when TC activity is reduced. Significant differences in the number of TCs between HFP and LFP are found in the typhoon season from July through October. The differences in TC activity between HFP and LFP are also found in the areal extent of TC genesis. The area of TC genesis in HFPs extends more to the east than that in LFPs. Seasonal conditions of sea surface temperature (SST), relative vorticity at 850hPa, divergence at 200 hPa and outgoing longwave radiation (OLR) are analyzed to discuss the differences in TC activity between the two kinds of periods. Both oceanic and atmospheric conditions differ between HFP and LFP with statistical significance. In HFP, the SST in the eastern WNP (east of 150°E) is higher, and the convective activity at the area from 10°N to 20°N is stronger, than in LFP. These results show that the atmospheric and oceanic circumstances for HFPs in the tropical and subtropical WNP enhance the genesis of TCs in comparison with those for LFPs. It is suggested that the variability of SST in the Pacific, and that of large-scale atmospheric circulation are the main causes of the interdecadal variability of TC activity.