Concentrations of carbon monoxide (CO) in air were analyzed from seven urban roadside (U-RS) stations in Seoul, Korea over an 11-year period (1998-2008). As noticeable changes in CO levels coincided with the Natural Gas Vehicle Supply (NGVS) program in the year 2000, its concentration data were examined between 1998-2000 (period I) and 2001-2008 (period II). The mean values of CO from all 7 stations for the whole study period were recorded as $1.19{pm}0.22$ ppm with large drops in its mean from $1.83{pm}0.43$ ppm in period I to $0.94{pm}0.21$ ppm in period II. As such, CO levels between the two periods undertook the mean reduction rate of $46.9{pm}16.2%$ (range of 18.3 to 59.9%). According to the analysis of longterm trends of CO, its concentrations in the U-RS stations decreased very rapidly with the annual mean reduction rate of 9.5% $yr^{-1}$. A line of evidence collected in this study suggests that the implementation of the NGVS program has been quite effective to induce the alterations in CO levels in all urban roadside locations throughout the city of Seoul.