Abstract: Statistic analysis is made of the characters of the mass concentrations and chemical compositions of PM_2.5 in Beijing, based on the observations during the period of 2003 to 2004. It is found that the mean concentration of PM_2.5 shows the lowest value in summer, while it reaches the maximum of year in winter and spring. Moreover, the daily average mass concentration of PM_2.5 in summer is 71 μg/m³, which is lower than that in other seasons of about 110 μg/m³. The yearly average mass concentration of PM_2.5 is 100 μg/m³, which is much higher than the U S Air Quality Standard for yearly average mass concentration of PM_2.5 of 15 μg/m³. Then, the relationship of PM_2.5 and PM₁₀ is discussed. The mean ratio of PM_2.5 to PM₁₀ is 0.55 for the whole year, which is close to the values in previous research for other cities including Guangzhou, Wuhan, Chongqing and Lanzhou in China. In addition, the mean ratio of PM_2.5 to PM₁₀ is 0.62 and 0.52 for the heating and non heating season respectively. It shows a slightly high trend in the heating period. Seasonal characteristics of the ratio of PM_2.5 to PM₁₀ is 0.3—0.6 in summer, 0.3—0.8 in spring and autumn, and 0.4—0.9 in winter. The results indicate that diurnal ratio of PM_2.5 to PM₁₀ responds to the meteorological conditions and the anthropogenic activities. Sand dust weather and daily traffic lead to the increase in concentration of coarse particles more rapidly than that of fine particles in atmosphere. As a result, the ratio of PM_2.5 to PM₁₀ decreases. On the other hand, the house heating in winter and the photochemical reaction in summer cause the increase in the ratio of PM_2.5 to PM₁₀ as well. Furthermore, the analysis to meteorological factors reveals that the change of the concentration of PM_2.5 is well related to pressure, relative humidity, and wind speed. In addition to the positive correlation with humidity, the mass concentration of PM_2.5 is negatively correlated with wind and pressure except in summer. Finally, the sources of PM_2.5 are analyzed by using the method of positive matrix factorization. It is found that SO₄^2-, NO₃^- and NH4+ are the primary water soluble ions in PM_2.5 in Beijing. Moreover, five sources of PM_2.5 in Beijing are identified. They are soil dust, coal combustion, traffic, sea salt aerosol and steel production. Compared with the results of previous research in sources identification of aerosol in Beijing, some conclusions are made. Firstly, soil dust and coal combustion have been the primary sources since 1980 s, while the contribution of traffic emission to fine particles has grown gradually from 1983 to 2001. Secondly, the effects of a number of sources including coal combustion, sea salt aerosol, biofuel combustion and second aerosol, on PM_2.5 vary with seasons. Thirdly, the process of transportation affects the composition of PM_2.5 distinctly, and the characteristic element of PM_2.5 from special sources related with the wind direction closely. Besides, the results of sources identification to PM_2.5 in Beijing area are different in different sites and periods. As a result, selecting representative sites and observing period for the study is very important.