Size-segregated aerosol samples were collected using Andersen cascade sampler from Sep. 2009 to Aug. 2010 in Xinglong, a regional atmospheric background station. The water-soluble inorganic ions were analyzed by IC. The result showed that the annual concentrations of the total water-soluble inorganic ions were (89.66 +/- 47.66), (54.44 +/- 34.08) and (44.39 +/- 29.95) microg x m(-3) in TSP, PM2.1 and PM1.1 respectively. SO4(2-), NO3(-), Ca(2) and NH4(+) were the dominant contributors of water-soluble inorganic ions. The total water-soluble inorganic ions in PM2.1 accounted for 61% of TSP. The total water-soluble inorganic ions in PM1.1 accounted for 50% and 82% in TSP and PM2.1, respectively. The seasonal variations of the total water-soluble inorganic ion concentration in TSP, PM2.1 and PM1.1 were the same, following the order of summer > autumn > springs > winter. The mean molar ratio of NH4(+) to SO4(2-) was larger than 2, indicating that NH4(+) was not completely neutralized by SO4(2-). NH4(+) -SO4(2-) and NH4(+) -NO3(-) concentrations were strongly correlated (R2 0.96 and 0.87), indicating that NH4(+) was mainly present as (NH4) 2SO4 and NH4NO3.