In order to analyze the characteristics and long-term trend levels of PM2.5 in Tainan City, Taiwan, Mesoscale Modeling System Generation 5 (MM5) and Community Multiscale Air Quality Model (MM5-CMAQ) modelling as well as box plots and time series analysis, were utilized in this study. The long-term trend analysis shows that the levels of PM2.5 (averaged at 38.3 µg m–3 and ranged between 33.1 and 41.9 µg m–3) in Tainan City for ten years (2005–2014) were above the yearly average standards of 15 µg m–3, showing non-attainment status. Overall, the results show a decreasing trend (from 41.9 µg m–3 in 2005 to 35.0 µg m–3 in 2014) in the levels of PM2.5 in Tainan atmosphere in the ten year period. The results of the MM5-CMAQ air quality modeling, indicate that the highest contribution on PM2.5 in Tainan City was from trans-boundary pollution from neighbouring cities (34.2%), while long-range transport and local emissions from Tainan each contributed a fraction of approximately 32.9%. In terms of local sources, the highest influence is from area sources (18.6%), followed by line sources (7.7%) and point sources (6.6%). Thus, to control PM2.5 in Tainan City, the focus should be on construction, road dust, and residential activities.