本文建構一個三國?廠的寡占市場模型，模型包含只消費的本國，和從事生產的低污染?國與高污染南國，且此產品的生產過程會產生跨國境的污染。文中分別分析在本國可自由課徵?國關稅、最惠國待遇原則下課徵統一關稅、與?國簽定區域貿?協定、與南國簽定區域貿?協定等四種情形下，最適關稅?與福?的分析與比較。文章並進一步研究在?國提高環保標準、污染防治技術進步與污染外溢效果加重等三種?況下，對南??國與對本國的影響。研究主要結果為：第一，??國廠商的污染防治比?對?國污染稅?的敏感?愈高，而?國政府提高環保標準，則?國?潤上升的可能性愈大。對南國而言，單一關稅下?潤增加，其他情形下，?潤?化的可能性提高。第二，?本國將環境品質納入福?函?中，採??同的貿?措施下之福?效果，最好的為對南、??國自由課稅，次佳為最惠國待遇下採用單一關稅，再其次為與?國自由貿?，最差者為與南國自由貿?。

This paper constructs a 3-country, 2-firm oligopoly model which consists of a home country with no production, a low-polluted north country, and a high-polluted south country. Trans-boundary pollutions to the home country are presumed. Optimal tariff and social welfare are analyzed under the following four cases: 1. the home country impose differential tariff, 2. union tariff to obey the most favored nation principle, 3. trade union with the north country, 4. trade union with the south country. In addition, the effects of increasing pollution tax by the north country, improved technology of pollution-abatement, and cross-border pollution increase on the profit and social welfare are studied. The results indicate that if the north firm’s abatement ratio is more sensitive to the pollution tax, then the north country will likely improve welfare by north government’s raise of pollution tax. As for the south country, the profit will be improved under uniform tariff situation and decreased under the other cases. For the home country, if environmental quality is considered in the welfare function, the welfare will be increased most by imposing differential tariffs, secondly by adopting uniform tariff, thirdly by free trade with the North, and least by free trade with the South.