This paper analyzes the benefits and losses associated with cooperation among ASEAN members in mitigating their CO2 emission, particularly by implementing a uniform carbon tax across ASEAN. To achieve this goal, this paper uses a multi-country CGE model for ASEAN, known as the Inter-Regional System of Analysis for ASEAN (IRSA-ASEAN) model. This study finds that the implementation of a carbon tax scenario is an effective means of reducing carbon emissions in the region. However, this environmental gain could come at a cost in terms of GDP contraction and reduction in social welfare, i.e. household income. Nevertheless, Indonesia and Malaysia can potentially gain from the implementation of a carbon tax as it counteracts price distortions due to the existence of heavy energy subsidies in these two countries.