本文利用產業內貿易模型探討垂直關係對於雙邊貿易國最適關稅政策的影響。本文建立一產業內貿易模型為兩國各別存在一下游廠商，並分別從第三國獨占廠商進口中間投入生產同質產品，最終於兩國國內彼此競爭貿易。本文利用三階段賽局求解，比較不同市場結構下影響最適貿易政策之差異來源，並比較第三國上游獨占廠商採行不同訂價策略時，影響最適貿易政策之差異來源為何。對於進口國政府而言，其貿易政策可使用進口從量稅或進口從量補貼。本文發現當產業結構不存在垂直關係時，在線性需求函數下兩國最適進口政策為課徵進口從量關稅，但是當產業結構存在垂直關係，不論上游廠商採行單一訂價或差別訂價，兩國之最適進口政策為大國必然課徵進口稅，而小國取決兩國之相對市場規模決定進口補貼或課稅，其中水平關係貿易模型之最適關稅大於垂直關係貿易模型單一訂價之最適關稅，再大於垂直關係貿易模型差別訂價之最適關稅。 We consider the effect of vertical relationship to the optimal import policy for the intra-industry countries. In this paper, we set up an intra-industry trade model which two firms import the input from a monopolist upstream at the third country and produce homogeneous final goods compete to each other. Hence, we solve the solution by three-stage game and compare the effects to optimal import policy under different market structure. In general, the import policy tools for a government are import tariff and import subsidy. Therefore, we find when vertical relationship does not exist, the optimal import policy under linear demand is import tariff. On the other hand, when vertical relationship exists, the optimal import policy for the large market size country is import tariff and for the small market size country depends on the relative market size of two countries. At last, we find the optimal tariff under horizontal relationship is larger than uniform pricing under vertical relationship than price discrimination under vertical relationship.
