PM2.5 and PM2.5-10 aerosol samples were collected in four seasons during November 2010, January, April and August 2011 at 13 urban/suburban sites and 1 background site in Western Taiwan Straits Region (WTSR), which is the coastal area with rapid urbanization, high population density and deteriorating air quality. The 10-days average PM2.5 concentrations were 92.92, 51.96, 74.48, 89.69 μg/m3 in spring, summer, autumn and winter, respectively, exceeding the Chinese ambient air quality standard for annual average value of PM2.5 (Grade II, 35 μg/m3). Temporal distribution of water soluble inorganic ions (WSIIs) in PM2.5 was coincident with PM2.5 mass concentrations, showing highest in spring, lowest in summer, and middle in autumn and winter. WSIIs took considerable proportion (42.2%~50.1%) in PM2.5 and PM2.5-10. Generally, urban/suburban sites had obviously suffered severer pollution of fine particles compared with the background site. The WSIIs concentrations and characteristics were closely related to the local anthropogenic activities and natural environment, urban sites in cities with higher urbanization level or sites with weaker diffuse condition suffered severer WSIIs pollution. Fossil fuel combustion, traffic emissions, crustal/soil dust, municipal constructions, sea salt and biomass burnings were the major potential sources of WSIIs in PM2.5 in WTSR according to the result of principal component analysis (PCA).