本論文利用三國貿易模型，探討經濟整合對會員國的關稅政策和福利之影響。本論文發現，如果市場為獨占且區外廠商邊際成本固定，兩進口國在成立關稅同盟後的福利均下滑，且兩國市場規模差異越大，整合後的福利下降越多。若此獨占廠商的邊際成本隨產量遞增，要使兩進口國在關稅同盟下的福利高於整合前的水準，只有在兩國的市場規模差異不大時才有可能發生。如果市場為寡占，自由貿易區的成立將引發貿易擴張，使會員國與非會員國福利均上升；而關稅同盟下會員國的福利比整合前或自由貿易區高的條件為兩國市場規模差距不大，且非會員國廠商較不具成本競爭優勢。 We construct a three-country model to study how economic integration affects optimal tariffs and the welfare of member countries. We find that, if the firm outside of the integrating bloc is a monopolist and has constant marginal cost, two importing countries would face reduced aggregate welfare after they form a customs union. In addition, the welfare is decreasing with the severer variance of market size between two union countries. On the other hand, if the monopolist has increasing marginal cost, the formation of customs union would improve the aggregate welfare of member countries when their markets not differ too much. Moreover, if there exist firms in the member countries to make the market oligopolistic, the formation of the free trade area would induce trade expansion and improves welfare of both member and non-member countries. However, only when the difference of market size between two member countries is small, and the firm outside the bloc has weak competitive advantage on cost, then the welfare under customs union is higher than the level under free trade area and pre-integration circumstance.
