This paper constructs the two-part tax－a combined form of output tax and entrance fee－for polluting oligopolists under endogenous entry. In the presence of external damage that varies exogenously with aggregate output, we show that the two-part tax produces the ex post Pigouvian rule and thus achieves the first-best optimum. We also examine a detailed analysis of the impact of the two-part tax on social welfare and government revenues. Finally, when estimation errors exist in the process of regulation, we identify the incentive conflicts between interest groups and analyze the effects of estimation errors on determining optimal tax. In particular, we show that if the regulator takes care of both welfare loss and revenue gain under the proposed two-part tax, not only over-estimation on the slope of external damage but also under-estimation on the slope of market demand should be taken into the policy consideration.