Abstract: A ground-based observation of fine particles (PM_2.5) is conducted in 2010 at Lin'an regional background station in the Yangtze River Delta region. Daily PM_2.5 samples are collected by Mini-Vol portable aerosol sampler on 47 mm quartz filters with the flow of 5 L/min. A total of 223 valid PM_2.5 samples are collected in 2010. The concentrations of Na⁺, NH₄⁺, K⁺, Mg²⁺, Ca²⁺, F^-, Cl^-, NO₃^- and SO₄^2- in PM_2.5 are determined by using Dionex 600 Ion Chromatography. EC and OC in PM_2.5 are determined by DRI Model 2001A Thermal/Optical Carbon Analyzer. The mass concentration of PM_2.5 at Lin'an Stationranges from 1.4 to 442 μg·m^-3, with the annual average concentration (58.2 ± 50.8) μg·m^-3 during 2010. Seasonal variation of PM_2.5 concentrations is significant. The concentrations of PM_2.5 are 70.3, 28.9, 66.2 μg·m^-3 and 77.4 μg·m^-3 in spring, summer, autumn and winter, respectively. To gain an insight into the impact of transport on PM_2.5 levels at Lin'an, air mass backward trajectories are calculated and analyzed in combination with corresponding pollutants concentrations using the HYSPLIT4 model. The results indicate that the site is under significant regional-scale influence of the long-range transport from the Yangtze River Delta region and northern China. The annual concentration of total water-soluble inorganic ions is (28.5±17.7) μg· m^-3, contributing an average of 47% of PM_2.5 mass concentrations. In PM_2.5, the concentrations of the most abundant ionic species following the order of SO₄^2-, NO₃^-, NH₄⁺, Cl^-, Na⁺, Ca²⁺, K⁺, F^- and Mg²⁺. Three major ions SO₄^2-, NO₃^- and NH₄⁺ account for 69% of the total water-soluble inorganic ions. The annual mean concentrations of OC and EC are (10.1±6.7) μg·m^-3 and (2.4±1.8) μg·m^-3, respectively. The concentrations vary in ranges with 0.8—29.8 μg·m^-3 for OC and 0.03—8.6 μg·m^-3 for EC. The average concentrations of OC and EC are highest in autumn and lowest in summer. The concentrations of OC and EC are (15.3±6.5) μg·m^-3 and (3.6±1.5) μg·m^-3 in autumn, while (4.8±2.2) μg·m^-3 and (1.2±0.6) μg·m^-3 for OC and EC in summer, respectively. OC and EC show a significant correlation, indicating that OC and EC are mainly from the same sources.