Traffic emission is the main air pollutant source in urban area. Since the sources are close to the pedestrian and people in commercial districts, the citizen’s exposure risk caused by the traffic air pollution source is concerned. Air quality has become one of important indicators of urban life quality. To evaluate the correlation between traffic air pollutants emission and air quality of Taipei metropolitan area, one need to include two administration districts, namely Taipei City and Taipei County. In this study a system dynamic theory based model for the assessment of Taipei transportation management on the metropolitan’s air quality is extended and revised. The extended model is more appropriate to reflect the current situation of transportation system and activities in Taipei metropolitan area than before. The model is validated and calibrated to confirm its assessment applications. Two ongoing infrastructures in the Taipei metropolitan area, namely the Bali-Xindian expressway and some extended MRT routes were modeled to assess their prospective impact on the city’s traffic activities and hence on the air quality. This study includes two models, trip analysis model and traffic emission assessment model. The trip analysis model is to model trips and routes in the metropolitan’s transportation network. The traffic emission assessment model integrates three sub-model by System Dynamics theory, namely the Logit sub-model, traffic flow sub-model and traffic emission sub-model. The models then were used to calculate and to compare the air pollutant emission change before and after the mentioned transportation infrastructures were accomplished. In the study, the model was confirmed by the processes of validation and calibration with government published data. The processes confirm the results were quite agreeable to the current traffic situation. The simulation results of the accomplishment of Bali-Xindian expressways, MRT routes of Xinyi, Neihu, Xinzhuang, Luzhou, Taoyuan international airport and Taipei Metro-Circular line indicated that the total emission of NOx , HC, and CO in Taipei metropolitan area is expected to be reduced, especially when the MRT routes are accomplished.