Fine particulate matters (PM2.5) have been identified as one of the major air pollutants in urban areas, which are responsible for the deterioration of the atmospheric air quality as well as adverse effects on public health. In this study, the mass concentration, water-soluble ionic component, trace metal component, carbon component and modeling the contribution source for PM2.5 was characterized for Chiayi City, which has high population density and surrounded by agricultural area. The lowest PM2.5 mass concentrations were registered in the summer (9–22 µg m–3), while for the spring, autumn, and winter were well above the healthy level suggested by World Health Organization (WHO). For all seasons, the dominants were the sulfate (SO42–), nitrate (NO3–) and followed closely by ammonium (NH4+). Those secondary aerosols were transformed from SO2 and NO2 into particulate NO3– and SO42– during spring, autumn and winter. Lower carbon mass concentrations were observed for summer (2.03–2.49 µg m–3) corresponding to the highest carbon content in PM2.5 mass concentrations in terms of percentages (average 18.1%). Using the Chemical Mass Balance receptor model, the secondary nitrate (NO3–), primary traffic source, secondary sulfate (SO42–), re-suspending soil particle, and petrochemical industry were identified as the major sources of PM2.5 in Chiayi City. Consequently, the PM2.5 contributions were complicated in a small but various seasons and geological distributing area. The air quality control strategies were thus seasonal and periodical dependent.