本文在雙占市場的架構之下，探討品質如何影響廠商的訂價，我們發現當高品質產品的品質提高時，兩廠商因為競爭程度下降，所以皆會提高訂價，而當低品質產品的品質提高時，高品質產品價格降低；低品質產品價格則會先升後降，取決於消費者願付價格提升的效果和競爭程度提高的效果的大小。在品質影響最適關稅的方面，若高品質產品的品質提升，則進口國政府將會提高最適關稅；若低品質產品的品質提升，則進口國政府的最適關稅不一定會提高或降低。我們也發現，在關稅內生時，高品質產品品質提高，會造成單一關稅下的低品質廠商利潤先升後降，而在不同關稅下的低品質廠商利潤則不會有此現象。因為高品質廠商的品質提高不僅降低了競爭程度，也使得進口國政府對其課徵的進口關稅會增加。最後，高品質廠商因較低的進口關稅而可能有意願到低品質國家直接投資，而低品質廠商因較高的關稅不會選擇到高品質國家直接投資。

This study analyzes how quality affects manufacturers’ pricing under the duopoly market structure. First, we found that when the quality of high-quality products increases, the two manufacturers will increase their pricing due to the decline in competition. When the quality of low-quality products increases, the price of high-quality products will decrease. The price of low-quality products will increase first and then decrease, depending on the effect of consumers' willingness to pay and the effect of increased competition. Furthermore, if the quality of high-quality products increases, the government of the importing country will increase the optimal tariff. If the quality of low-quality products increases, the government of the importing country may increase or decrease the optimal tariff depending on different effect. In addition, we have also found that when the tariff is endogenous, the quality of high-quality products increases, which will cause the profits of low-quality manufacturers under a single tariff to rise first and then decline. While the profits of low-quality manufacturers under different tariffs will not have same result. That is because the quality improvement of high-quality manufacturers not only reduces the degree of competition, but also increases the import tariffs imposed by the government of importing countries. At last, we observe that the high-quality manufacturers may be willing to invest directly in low-quality countries due to lower import tariffs, while low-quality manufacturers will not choose to invest directly in high-quality countries due to higher tariffs.

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