The Taipei main station is a major transfer station in the Taipei Rapid Transit System and is located at the center of Taipei metropolitan areas. This study investigates particle mass concentrations and size distributions at the concourse in this underground station using an optical particle counter. On-site measurements were taken during January–February 2008. Experimental results show that PM10 and PM2.5 levels in the Taipei main station were 9.83–104.26 μg/m3 and 3.84–59.74 μg/m3, respectively. The lognormal mass size distribution in the Taipei main station had two modes; one near 0.27 μm and the other at about 12.5 μm. Additionally, the mean mass concentrations were governed by particles with coarse PM. Measurement results also suggest that average PM10 and PM2.5 levels in the indoor station were about 0.70 and 0.53 times those outdoors, respectively. The PM levels in the indoor station and outdoors were positively correlated, indicating that PM levels at the concourse in the Taipei main station are significantly influenced by outdoor ambient PM levels. Moreover, the low PM2.5-to-PM10 ratio at the concourse in the Taipei main station was likely the result of coarse PM being re-suspended in the station concourse due to passenger movement.