本文建立一個三國兩廠商的進口模型，將兩廠商間競爭的猜测變量納入考慮以探討進口國 之最適差別性關稅政策，在模型中兩來自不同國家之外國廠商將其生產之產品出口到進口國從 事數量競爭，進口國則對兩外國廠商之產品課徵差別性從量關稅。本文發現，當兩外國廠商之 產品為同質時，若兩廠商的競爭的猜测行為類似於完全競爭時，則進口國對兩廠商課徵之關稅 稅率相等；反之，若兩廠商之競爭行為非完全勾結或完全競爭，則進口國會對低(高)成本廠商課 徵高(低)關稅。當兩外國廠商之產品不完全同質時，若兩廠商之競爭行為較勾結，則進口國會對 低(高)成本廠商課徵低(高)關稅；若兩廠商之競爭行為較競爭，則進口國會對低(高)成本廠商課 徵高(低)關稅。

This paper sets up three country two firm model where two firms in different country export their product to engage in quantity competition in the host country that imposes discriminatory tariff rate on the two firms. We find, when the two exporting goods are homogenous, if the behavior of the two firms displays a perfect competition behavior, the optimal tariff rates on the two firms are equal, otherwise, the optimal tariff rate on the low cost firm will be greater than that on the high cost firm, and when the two exporting goods are heterogeneous, if the behavior of the two firms are more collusive, the optimal tariff rate on the low cost firm will be less than that on the high cost firm.