本文在異質性Bertrand及Cournot兩種競爭模式下，探討關稅減讓政 策對於最適污染稅與社會福利水準的影響。本文發現：（1)當產品為完全 異質時，價格競爭的模式與數量競爭模式所獲致的結論相同；當產品為不 完全異質時，不同的競爭模式有明顯不同的結果。（2)不管是在價格競爭 抑或是數量競爭的情況下，關稅減讓政策對於最適污染稅的影響與產品的 異質性有關，當產品為完全異質時，關稅減讓政策會提高最適的污染稅； 當產品的異質性較低時，則關稅減讓政策對於最適污染稅的影響則不明 確。此外，關稅減讓政策對社會福利的影響效果並不明確。（3)在大部分 的情況下，價格競爭的關稅減讓對汙染稅與社會福利的提升效果皆大於數 量競爭的情況。

This paper uses Bertrand competition and Cournot competition models with product heterogeneity to revisit the impact of tariff concessions on the optimal pollution taxation and social welfare. First, when the products are complete heterogeneity, the result of Bertrand competition is consistent with that of Cournot competition, whereas when the products are incomplete heterogeneity, the result of Bertrand competition is different from that of Cournot competition. Second, regardless of Bertrand competition or Cournot competition, when the products are complete heterogeneity, a decrease in a tariff leads to an increase in the optimal pollution taxation. Conversely, when the product heterogeneity becomes smaller, the effect of tariff concessions on the optimal pollution taxation is ambiguous. In addition, when the products are complete heterogeneity or incomplete heterogeneity, the effect of tariff concessions on the social welfare is ambiguous. Third, in most cases, it is obvious under Bertrand competition that a decrease in a tariff leads to increases in both the optimal pollution taxation and social welfare compared to Cournot competition.