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PM10concentrations were measured at four monitoring sites at the Daechaung station of the Seoul subway. The four locations included two tunnels, a platform, and a waiting room. The outside site of the subway was also monitored for comparison purposes. In addition, the effect of the platform screen doors (PSDs) recently installed to isolate the PM10in a platform from a tunnel were evaluated, and a comparison between PM10levels during rush and non-rush hours was performed. It was observed that PM10levels in the tunnels were generally higher than those in the other locations. This might be associated with the generation of PM10within the tunnel due to the train braking and wear of the subway lines with the motion of the trains, which promotes the mixing and suspension of particulate matter. During this tunnel study, it was observed that the particle size of PM10ranged from 1.8 to 5.6 μm. It was revealed that the PM10levels in the tunnels were significantly increased by the PSDs, while those in the platform and waiting room decreased. As a result, in order to estimate the effect of ventilation system on PM10levels in the tunnels, fans with inverters were operated. It was found that the concentration of PM10was below 150 μg/m3when the air flow rate into a tunnel was approximately 210,000–216,000 CMH.