The two flights aerosol data sampled with Passive Cavity Aerosol Spectrometer Pr obe PCASP100X probes of an airborne Particle Measurement System ( PMS ) in Apr il 2 and April 17, 2005 over Beijing were analyzed. The result shows that the ma ximum of number concentrations of aerosol in those two days were 1422.7cm-3  and 2142.1cm-3. The mean diameters were 0.22μm and 0.25μm, re spectively. There were a strong gradient of number concentration of aerosol over Shahe Airport. Temperature inversion and other meteorological factors, such as the planet boundary layer, had important influence on the number concentration o f aerosol and mean diameter. Location also has an important impact on aerosol.