Fine particulate matters (PM2.5) has been identified as one of the major air pollutants in urban areas, which are responsible for the adverse effects on public health and the deterioration of visibility. New PM2.5 air quality standards were promulgated in Taiwan on 14th May 2012, as well as the standard sampling and analytical method for atmospheric PM2.5 (NIEA A205.11C) on 24th April 2012. In this study, the atmospheric levels and characteristics of PM2.5 in Tainan during 2013 were evaluated by measuring the mass concentration of PM2.5 and analyzing the water-soluble ionic, carbon, and metal components. Additionally, a Chemical Mass Balance (CMB) receptor model was used to identify possible sources of PM2.5 and their contributions. Based on results of this study, the current PM2.5 levels in Tainan in spring and winter (41–49 µg m–3) were substantially higher than the yearly average PM2.5 air quality standards (15 µg m–3). According to chemical composition analysis, secondary aerosols (NH4+, NO3–, and SO42–) contributed approximately 50% and 60% of PM2.5 mass in spring and winter respectively; but were responsible about 40% by mass in summer at both Tainan and Xinying stations. From the results of CMB model, the main contribution sources to the PM2.5 in Tainan are traffic emissions (31.5%), ammonium sulfate (25.5%), ammonium nitrate (12.5%), and crustal elements (11%). Consequently, to improve PM2.5 of Tainan City, the priority control pollutants (or sources) are primary PM2.5 (open burning, construction sites and road dust by vehicles), NOx (diesel vehicle emissions), and SOx (fuels).