本文利用特殊生產要素模型，探討一國既可以影響商品交易的貿易條件，也可以左右要素移動時外資的國外報酬率時，外資所得稅率（關稅率）的決定以及其與最適關稅稅率（外資所得稅率）的關係。分析結果指出：最適外資所得稅率受外國對本國出口品價格的需求彈性，以及外資供給彈性的影響。此兩項彈性值愈大，最適的外資所得稅率愈低。而最適關稅率與最適外資所得稅率的關係則為同向變動。甚至訂定的關稅並非最適關稅時，次佳外資所得稅率也與關稅率成正向關係。而在關稅率不是很高時，政府有可能應採取給予外資補貼的政策，以鼓勵國外資本的流入，進而擴張進口財產業。至於次佳的關稅率是高於或低於最適關稅率，則視既定的外資所得稅率是高於或低於最適稅率水準而定。

　　　　　This paper investigates the optimum tariff and taxation of foreign capitalin an open economy which owns monopoly power in its exportables and imported capital. A specific-factor model is utilized and the main findings are: (1) the optimal rate of foreign capital taxation changes directly with the foreign demand elasticity of exportables and the supply elasticity of imported capital; the optimal tariff rate changes inversely with the foreign demand elasticity of exportables. (2) the second-best tax rate on foreign capital varies directly with the given rate of tariff; vise versa for the second-best tariff rate.