Ultrafine particulate matter in the Shanghai subway system, we measured the PM concentrations at the Songyuan Road and the Jiao Tong University stations in Line 10, at the Caoyang Road station in Line 11, and inside trains on Shanghai metro Line 10. The PM2.5 and PM1.0 values averaged 39.7 and 27.6 μg/m3 at the stations, and 24.4 and 17.5 μg/m3 inside the trains. Obviously, the PM2.5 and PM1.0 concentrations inside the trains were lower than those on the platforms. The PM2.5 and PM1.0 concentrations on the platforms and in the train compartments were both positively correlated with values in the ambient air. PM concentrations on the platforms were influenced by the piston wind created by the approaching trains, and increased similarly whether westbound trains or eastbound trains were entering the station. Moreover, the effect of the piston wind varied among locations on the platform; the effect of the piston wind was stronger at both ends of the platform than in the middle. The PM concentrations inside the trains were also influenced by opening of the train doors. The PM concentrations increased during opening of the doors, and the increase was more remarkable for PM2.5 than for PM1.0. PM2.5 and PM1.0concentrations were strongly and significantly correlated, suggesting that the particles are derived from the same sources.