This study was performed to investigate the concentration levels of CO2 and the associated parameters in the subway from lines 1 to 8 in Seoul metropolitan area, during 1 to 30 Sept. 2005. CO2 concentration was measured at the entrance and center in subway passenger cabin by non-dispersive infrared analyser. The associated parameters for CO2 were estimated based on the number of passenger, open and closed doors etc. The mean CO2 concentrations were measured to be 1,646±712 ppm in subway passenger cabins. The CO2 concentrations showed the highest values between 7 and 9 AM in rush hour(p〈0.01). The correlation coefficient between CO2 and the number of passenger was highly significant(r=0.824). The general linear model indicated that subway line, subway location (ground and underground tract), and measurement point (enterance and center of cabins) and running time (morning and afternoon) significantly influenced the concentrations of CO2. An extended study is needed to examine the sources of CO2 in subway cabins and to compare pollutants concentration among subway lines.