本論文建立一個第三國市場中的同質產品 Cournot 競爭模型。模型中，本國有低成本與高成本兩類廠商，外國也有廠商共同出口到同一進口國市場。本國福利定義為本國廠商在該市場中的出口利潤總和。本論文先處理三廠商模型，說明需求、成本、關稅與固定成本如何進入廠商的利潤函數，再推導三家廠商在 Cournot 競爭下的產量、價格與利潤。接著，本論文利用這些結果討論廠商關稅提高的影響，並在一般化分析中檢查成本相對高低、廠商家數以及 FTA 與 WTO 架構對結論的影響。 研究結果顯示，若只看短期產量為正條件，廠商通常要等到產量降為零才退出。此時關稅提高會使本國廠商的產量與利潤減少，本國福利大致呈現分段下降，門檻處也多半是連續銜接。然而，若將固定成本納入長期退出判斷，廠商會依據扣除固定成本後的利潤是否為正來決定是否留在市場。這時高成本廠商可能在產量仍為正時提前退出，市場也會從三家廠商競爭改為兩家廠商競爭。當本國低成本廠商承接的市場份額所帶來的利潤增加，大於退出廠商原本的利潤貢獻時，本國福利便可能在門檻點上升。 本論文的重點不在於主張關稅一定有利，而是指出關稅效果不必然是單調的。在成本不同、固定成本存在、且廠商會因利潤不足而退出的產業中，關稅會同時造成成本壓力與市場重整。政策效果要看退出的是哪一類廠商、留下來的廠商是否有能力承接市場份額，以及承接後的利潤是否留在本國廠商手中。;This thesis develops a homogeneous-goods Cournot model in a third-country market. Domestic and foreign firms export to the same importing market， while the home country contains both a low-cost and a high-cost firm. Home welfare is defined as the sum of domestic firms’ export profits in that market. The analysis first sets up the three-firm benchmark and derives the corresponding output， price， and profit conditions. It then studies the thresholds at which firms leave the market and examines whether home welfare may rise after market structure changes. Further extensions consider cost ranking， the number of firms， and the FTA and WTO frameworks. The main result is that the welfare effect of a tariff depends on the rule that determines which firms remain active. Under a short-run output rule， a firm exits only when its equilibrium output falls to zero. In that case， a higher tariff generally lowers domestic output and profits， and home welfare is mostly piecewise decreasing. In contrast， when fixed costs are incorporated into the long-run exit decision， firms may exit once their net profits fall to zero. A high-cost firm may then leave the market before its output becomes zero. If the low-cost domestic firm expands enough after this exit， the gain in its profit may exceed the lost profit of the exiting firm， and home welfare may rise at the threshold. The paper does not claim that tariffs are beneficial in general. Rather， it shows that tariff effects may be non-monotonic when firms differ in costs and when market structure responds to profit conditions. The relevant question is not only how tariffs change costs， but also which firms exit， which firms absorb the released market share， and whether the resulting profits remain with domestic firms.
