The NCEP/NCAR reanalysis, outgoing longwave radiation (OLR) and tropical cyclone datasets are employed to investigate the clustering characteristics of tropical cyclone activities over the western North Pacific from June to September in 1991 and their associations with intraseasonal oscillations (ISOs), in which tropical cyclone data refer to the best tracks from the Shanghai Typhoon Institute of China Meteorological Administration. Tropical convective activities are found to exhibit an intraseasonal oscillation with a period of 20－60 days during the summer of 1991 over the western North Pacific. Within this region, tropical cyclone occurrences are modulated by a similar period, i.e., tropical cyclone activities show features of periodicity and clustering over there. The tropical cyclones are prone to occurring frequently in the form of a cluster during the wet phase of the ISO, because anomalous cyclonic circulation at the lower levels forms a favorable background circulation for the generation and development of tropical cyclones. The unstable westerly jet on the southeastern side of such an anomalous cyclone in the lower troposphere induces frequently some disturbances, and the amplitudes of these initial disturbances increase due to positive vorticity anomalies. Meanwhile, positive genesis potential anomalies of tropical cyclones during the wet phase of the ISO indicate that the favorable vertical shear conditions are satisfied for the development of tropical cyclones, leading to the clustering of tropical cyclone.