To reduce emissions of Greenhouse Gas (GHGs) from automobiles, real on-road emission data should be required to help policy makers establish standards for reducing GHG emissions. Since general studies on calculating $CO_2$ emissions from transportation sources have been based on petroleum consumption data, it has been difficult to analyze spatial activities and characteristics of transportation sources. In this study, we propose a method for calculating $CO_2$ emissions from on-road transportation sources in Seoul. We focused on $CO_2$ emission calculation by applying real traffic flow data and analyzed base emissions from the main roads and local streets. Because the emissions were calculated using a 1 km${ imes}$1 km grid cell format, these data can be applied to other compatible transportation data sets for air pollution analysis, modal shift analysis, etc in the transportation sector.