本論文的目的在於，以一經濟理論模型探討本國政府加入國際貿易協定，
面對關稅降低，與奢侈品進口商及國產廠商在市場進行競爭下，制定最適奢侈稅稅率。本論文得到的結論如下。
一般而言，進口產品降低關稅將會使國產品交易數量下降，可能造成人民
可能過度奢侈消費。因此，本國政府針對消費者購買超過法定金額的進口奢侈品課徵奢侈稅。這就代表政府在廠商在市場進行 Bertrand 價格競爭或 Cournot 數量競爭下，制定最適奢侈稅，以極大化社會福利將是本論文探討的課題。
本論文假設市場有兩廠商，進口代理商進口的奢侈品為高品質產品，國產
廠商生產的國產品為低品質產品，形成雙占市場。市場型態有未涵蓋與涵蓋市場，兩廠商面對奢侈稅在未涵蓋市場進行 Bertrand 價格競爭或 Cournot 數量競爭與涵蓋市場進行 Bertrand 價格競爭決定價格或銷售量。
最後結果可以發現，奢侈稅上升對廠商而言，代理商收取的價格下降與國
產廠商收取的價格上升，兩產品的市場價格都會上升，奢侈品銷售數量下降與國產品產量上升，總銷售量減少。對於社會福利而言，因應貿易協定調低關稅，關稅越低奢侈稅制定越高，以極大化調整後的社會福利，這就表示奢侈稅與關稅有替代關係。
在未涵蓋市場，奢侈品進口價格提高會使最適奢侈稅上升，然在涵蓋市場，
奢侈品進口價格上升則會使最適奢侈稅下降。在兩種市場型態下，政府越重視
社會分配，最適奢侈稅越高。特別注意的是，在最適奢侈稅下，兩廠商進行
Bertrand 價格競爭的社會福利較 Cournot 數量競爭下的高，且僅有廠商收取的價格、奢侈品進口商利潤及政府稅收會因關稅與奢侈稅而變化。

This thesis builds an economic theoretical model to analyze how the domestic government determines the optimal luxury tax rate while participating in international free trade agreements, and the competition between luxury goods import agent and domestic manufacturer in the market.
Generally, reducing tariffs on imported goods decreases the sales of domestic products, and results in that domestic consumer could purchase luxury goods excessively.Consequently, the government imposes a luxury tax on imported luxury goods exceeding a statutory amount. This paper explores how the government
maximizes adjusted social welfare by setting an optimal luxury tax rate under Bertrand
competition or Cournot competition in the market.
The model assumes a duopoly, luxury goods import agent and domestic manufacturer. luxury goods import agent imported high-quality luxury goods, and domestic manufacturer produces low-quality domestic goods. Two firms compete in
the market by Bertrand competition or Cournot competition. Additionally, we assume the market structure, uncovered and covered market. In sum, facing the luxury tax, they
compete in the uncovered market by Bertrand competition or Cournot competition and in the covered market by Bertrand competition.
We find that increasing the luxury tax rate raises market prices, decreases luxury
goods sales, and increases domestic product output in both market structure. Moreover,
the policy results in the reduction of overall sales in the uncovered market.AS for social
welfare, after joining the international trade agreement, government need to decline the
tariff. The results indicates that the less tariff, the more luxury tax domestic government
need to enact.
Finally, the more imported price of luxury goods, the more optimal luxury tax in the uncovered market. However, the higher import prices would reduce the optimal luxury tax in the covered market. In both market structure, if considering the social distribution, the domestic government levies more luxury tax. Notably, Bertrand price competition yields greater adjusted social welfare than Cournot quantity competition under the optimal luxury tax rate.

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