本文以 Barnett (1980) 一文為基礎，採用第三地市場以納入廠商型態為多國籍企業 (multinational enterprise) 的模型。本文旨在分析當兩國政府面對獨占廠商的生產不效率及製造過程中汙染排放引發的生產外部性時，如何制定最適汙染稅稅率。並比較經濟開放後資本要素可自由於兩國間移動，最適汙染稅稅率會如何變化。 本研究發現政府課稅將受三種效果影響：消費者剩餘效果 (consumer surplus effect)、利潤移轉效果 (profit-shifting effect)、汙染控制效果 (pollution control effect)。不論在封閉經濟體系或開放經濟體系下，當消費者剩餘效果大於利潤移轉效果時，政府會將最適汙染稅率制定低於皮古稅 (Pigouvian tax)，以補貼獨占廠商增加產量以提高社會福利；若利潤移轉效果大於消費者剩餘效果時，最適汙染稅稅率將高於皮古稅，以藉由汙染稅獲取更多的企業利潤。 當經濟開放後資本可於兩國間自由移動，除原先稅率對產量的直接效果(direct effect) 外，稅率亦會透過資本間接影響產量，為間接效果 (indirect effect)。間接效果將使產量對於稅率的變動較不敏感，因此為吸引具流動性的資本要素進入當地投資，政府會將汙染稅稅率調高以獲取更多企業利潤。 / Based on the setting of Barnett (1980), the thesis develops a model to analyze the optimal pollution tax on multinational enterprises with negative externalities. Governments have only one policy variable: the pollution tax. Each government levies the pollution tax on the output of multinational enterprises, and the tax revenues are returned to the residents in a lump sum manner. The purpose of this thesis is to explore the optimal taxation on the multinational enterprise with externalities and mobile capital flow. The thesis obtains several results: First, no matter in the case of closed economy or open economy, the second-best optimal pollution tax may be less or greater than the marginal pollution damage. It may be lower because the consumer surplus effect dominates the profit-shifting effect, so the government chooses a lower tax rate to stimulate production and to increase social welfare. Second, when the capital can freely move between countries, the governments will levy a tax rate higher than that of a closed economy. Doing so can attract the flow of capital so that government can get more profit.