This paper analyzes the environmental impact and economic effect of introducing the Renewable Portfolio Standard (RPS) into Korean electricity market using the DECADES (Database and Methodologies for Comparative Assessment of Different Energy Source for Electricity Generation) model, a comparative assessment tool developed by IAEA. A bottom up approach is adopted for the evaluation of air pollutant emission and its impact of several RPS scenarios. The environmental damage costs of RPS scenarios are evaluated based on the Extern-E results and Thailand externality study carried by EGA T. The results of this study can be applied in determining or analyzing the national electricity policy and energy policy