Number concentrations of aerosols in 5 radius ranges (0. 15-0. 25 μm, 0. 25-0. 5 μm, 0. 5-1. 5 μm, 1. 5-2. 5 μm, 2. 5-5. 0 μm) were observed at Zhangye in northwestern China during the period from March 1991 to October 1992 by using an optical particle counter. The averaged number concentration of coarse particles (radius γ&ge ; 1. 5 μm) showed a seasonal variation that had a large maximum in spring and a large minimum in summer. In addition, a small maximum in late fall and a small minimum in winter were also observed. Fine particle (γ < 1. 5 μm) concentration shows a maximum in winter and a minimum in summer. The correlation between coarse particles (γ &ge ; 2. 5 μm) concentration and wind speed based on daily averaged data was not so high. However, the seasonal variation of coarse particles could be explained mainly by the variation of average wind speed. Almost no relation was found between aerosol concentration and wind direction. The concentration of aerosols was also influenced by precipitation. Dust-storm events were recorded in all seasons except mid-winter, and the appearance frequency of dust storm was highest in spring. During the dust-storm events, the concentration of coarse particles (γ &ge ; 1. 5 μm) increased by approximately one order of magnitude as compared with that before the event, along with a small decrease in fine particle (γ < 0. 5 μm) concentration.