More than 7.1 million people use the Seoul Metropolitan Subway (SMS) daily in 2008 and the number is increasing annually. The management of indoor air quality (IAQ) is getting more important according to the increasing concern of health and environment. The Ministry of Environment of the Republic of Korea established the recommended standard on the IAQ of public transportations - Guidelines on the management of indoor air quality of public transportations - in 2007. The guidelines prescribe the recommended standard about particulate matter less than 10 um (PM10) and carbon dioxide (CO2) during rush hour and non-rush hour in public transportations. In this study, we measured the levels of PM10 and CO2 concentrations in the subway cabins and counted the number of passengers to figure out the correlation with PM10 and CO2. Total 17 lines are being operated in SMS including 9 object lines of this study. The levels of PM10, CO2 and the number of passengers in a cabin were measured during the rush and non-rush hours. The equipments for measuring the PM10 and CO2 were placed on the middle of cabin. The measurement campaign was conducted for autumn seasons; October 14 to 31, 2008. The measurements performed for this study show that CO2 and PM10 levels were found to be lower than the recommended standards. However, measured CO2 levels were found to change depending upon increased/decreased passenger numbers during operation and over one section it was deduced that the level could exceed 4000 ppm with future increases in passenger numbers.