Abstract: In order to understand the characteristics and chemical compositions of PM_2.5 in the urban area of Beijing, 268 groups of PM_2.5 samples were collected from July 2012 to August 2013 at Dingling (background site), Yufa (regional transmission site), Gucheng, Dongsi and Tongzhou (urban sites). The samples were analyzed for water-soluble ions, organic and elemental carbon and inorganic elements. The average concentrations of OC, NO₃^-, SO₄^2-, NH₄⁺, EC, Cl^-, Si, Ca, Al and K⁺ in the samples were all above 1.0 μg/m³. The average concentrations of OC, NO₃^-, SO₄^2- and NH₄⁺ were (22.2±17.1), (21.5±25.9), (19.8±23.7) and (14.3±16.8) μg/m³ respectively, accounting for 17.9%, 17.3%, 15.9% and 11.5% of the PM_2.5. The concentrations of all species in the samples from non-background sites were higher than the samples from the background site. The concentration differences and chemical compositions among the three urban sites were not significant, but were quite different from those of Dingling and Yufa. OC, NO₃^-, SO₄^2- and NH₄⁺ were the most abundant species in the urban PM_2.5. The sum of the four species accounted for 62.5%, 54.2%, 46.0% and 62.7% of PM_2.5 in the spring, summer, autumn and winter, respectively. The species with the highest concentration was NO₃^- in the spring, SO₄^2- in the summer and OC in the autumn and winter. The value of OC/EC (5.7) showed a higher level of secondary organic carbon in the urban samples. The fractions of SNA and the ratios of NO₃^-/SO₄^2- increased with the AQI. The average ratio of NO₃^-/SO₄^2- in urban area was 1.14, larger than previous studies in Beijing. There are signs that the sources of PM_2.5 are gradually shifting from fixed sources to a combination of fixed and mobile sources in Beijing.