Suspended particles of PM2.5 in air were sampled concurrently at an urban site and a rural site in Pingtung County in southern Taiwan, in the spring, the summer and the fall of 2005. All samples were analyzed to identify eight water-soluble ions, carbonaceous contents, and 19 metal elements. Measurements reveal that the overall means of PM10 (and PM2.5) are 59.2 (47.4) μg/m3 at Pingtung (urban) site, and 63.6 (45.7) μg/m3 at Chao-Chou (rural) site. Although both sites exhibited strong correlations (R = 0.98 at Pingtung, and R = 0.78 at Chao-Chou) between PM10 and PM2.5 masses, the mean PM2.5/PM10 ratio was 0.81 at Pingtung, higher than 0.68 at Chao-Chou, suggesting that relatively large bare lands and outdoor burning on farms may have caused more coarse particles to be present in PM2.5 at a rural site (Chao-Chou) than at an urban site (Pingtung). Results of CMB (chemical mass balance) modeling show that the main contributors to PM2.5 mass at Pingtung are vehicle exhaust (49.3–62.4%) and secondary aerosols (SO42–, NO3– and NH4+) (31.2–37.8%), while those at Chao-Chou are the outdoor burning (25.3–50.4%) of agricultural waste, secondary aerosols (27.2–34.3%) and vehicle exhaust (12.0–26.9%), depending on the seasons.