本文旨在討論當生產造成環境汙染且廠商間生產效率不對等下，外人直接投資 (foreign direct investment, FDI) 對本國政府之最適汙染稅的影響。且於此最適汙染稅下，外國廠商如何決策是否來本國外人直接投資。本文假設市場上有兩家廠商，一為本國，一為外國。兩家廠商進行 Cournot 數量競爭。 本文發現當本國與外國廠商生產效率非對稱時，若外國廠商的生產成本較本國廠商為高，則本國政府會於較高的邊際汙染時，策略性地提高最適汙染稅。在面臨高額的汙染稅下，外國廠商較易退出本國市場，以達到本國政府阻擋外國廠商來本國外人直接投資的目的；反之，若外國廠商的生產成本較低，則本國政府會於較高的邊際汙染時，策略性地提高最適汙染稅，將利潤從外國廠商轉移至本國。而本國廠商將因高額的汙染稅退出本國市場。此時，外國廠商將獨占本國市場。若外人直接投資固定成本減少時，則本國政府將策略性地降低汙染稅，以吸引兩廠商皆留在本國市場。 當一般化此一模型時，隨著本國廠商家數增加，在外國廠商邊際利潤遞減下，本國政府的最適汙染稅會由一開始的逐漸減少，爾後反轉增加。上述的發現均推導出與過去相關文獻較為不同的觀點。 ;In this paper, we analyze the effect of foreign direct investment (FDI) on government’s optimal pollution tax in the presence of pollution and asymmetric efficiency between firms. And how foreign firm decide its foreign direct investment strategy under the particular pollution tax. We use a two-country setting (home and foreign) share game model. Each country has one firm, and two ﬁrms engage in Cournot competition. We show that when two firms are efficiency asymmetric, if marginal cost of foreign is higher than home firm, home government will strategically raise its optimal pollution tax to prevent foreign firm from doing foreign direct investment under higher marginal pollution. On the contrary, if marginal cost of foreign is lower than home firm, home government will strategically raise its optimal pollution tax to shift the profit from foreign firm to home country under higher marginal pollution. At this time, foreign firm will monopoly the home market. When fixed cost of the foreign direct investment decrease, home government will strategically reduce its optimal pollution tax to keep both firms in the market. After generalizing this model, we found that as the amount of firms in home country increasing, due to the decreasing marginal revenue of foreign firm, optimal pollution tax will star decrease and then return to increase. What we have found above are different to the related reference in the past.