The PM10 and PM2.5 levels have been measured inside trains on four routes and on platforms at five stations in the Taipei Rapid Transit System using TSI Model 8520 DustTrak monitors. In this study, the TSI DustTrak was calibrated with a Met One E-BAM sampler within the metro station. Experimental results suggest that the TSI DustTrak overestimated PM10 and PM2.5 levels by about 2.0 and 2.2 times, respectively, compared with those measured by the Met One E-BAM sampler. Measurement results show that average PM10 levels inside trains and on station platforms were 10–97μgm−3 and 11–137μgm−3, respectively, and average PM2.5 levels were 8–68μgm−3 and 7–100μgm−3, respectively. Moreover, experimental results demonstrate that PM levels inside trains and on platforms were lower than those measured for other subway systems worldwide. Additionally, the PM10 and PM2.5 levels measured by ambient air-quality monitoring stations near selected metro stations correspond to PM levels in the urban atmospheric environment in Taipei. Statistical results demonstrate that average PM10 and PM2.5 levels in metro stations were about 0.65–1.53 times and 0.89–1.75 times, respectively, those for urban ambient air. The PM levels inside stations and outdoors are positively correlated, indicating that PM levels in the metro system are influenced significantly by outdoor ambient PM levels.