In order to analyze the pollution level and the source of organic matter in atmospheric particulates in Beijing city, the authors had researched on the concentrations of organic matter, the group components, the compositions and distribution of aliphatic from PM10 and PM2.5 sampled in Beijing city during summer in 2005. The tracing study on pollutant source using some molecular geochemical parameters was conducted; The characteristics and distributions of the carbon isotopic compositions of n-alkane series from atmospheric particulates in Beijing city during summer have been studied as well. The results are concluded as follows: the concentrations of organic matter in atmospheric particulates in Beijing city during summer were high, the saturated hydrocarbons from each sampling area may be derived from both modern higher plants and incomplete combustion of fossil fuels to some extents; the distribution of hopanes clearly showed the impact of gasoline combustion exhausts; the distribution of steranes suggested that the influence of coal burning and engine lubricants on them was similar; the δ13C values of n-alkane series with low carbon numbers increase from cleaning zone, traffic zone, business zone to industry zone because of the influence of different pollutant source.