要旨：I develop a monopolistic competition model with pollution to analyze the effects of environmental policy on the average productivity of a country. In the model, firms are heterogeneous in their productivity. Given a particular distribution function of technology, I show that a stricter environmental policy will increase average productivity, and will have positive effects on the marginal decrease of profits and environmental damage. In addition, I show the optimal tax rate in a closed economy. In an open economy model, the environmental policy of a country will affect the average productivity of a trading partner. In other words, a stricter environmental policy in a home country leads a decrease in the productivity of a foreign country.