本文建構一個納入外國廠商出口競爭與進口關稅之開放經濟混合寡占模型，探討公營廠商目標差異對利潤稅中立性之影響。本文透過設定政策虛擬變數，於單一分析架構下比較Matsumura (1998)傳統公營廠商模型與Chang et al.(2018)公共社會企業(Public Social Enterprise, PSE)模式，並分析利潤稅、民營化政策與進口關稅間之策略互動關係。
研究結果顯示，在傳統公營廠商模型下，政府得以透過調整最適民營化程度，完全抵銷利潤稅變動所造成之產出扭曲，因此利潤稅具備中立性。相較之下，在 PSE 模式下，由於公營廠商之目標函數排除本國民營廠商利潤，利潤稅將直接影響公營廠商之產量決策，使其與民營化政策間之完全替代性質失效。然而，只要政府得以內生調整民營化程度，仍可將市場總產量維持於社會福利極大化之最適水準，使利潤稅變動之福利效果被最適民營化政策完全吸收。因此，即使公營廠商之經營目標由傳統模式調整為 PSE 模式，利潤稅中立性仍具穩健性。
此外，本文發現，在政府得以最適化民營化政策之條件下，進口關稅之福利效果獨立於國內利潤稅率之變動，顯示兩項政策間具有策略獨立性。提高進口關稅雖有助於攔截外國廠商利潤外移並增加關稅收入，但過度保護亦可能因本國公營廠商擴張產出，在邊際成本遞增下造成生產無效率。因此，最適關稅政策應在攔截利潤外移與維護國內生產效率間取得平衡。本文之研究結果擴展既有混合寡占市場與利潤稅中立性之理論分析，並為政府於公營事業目標轉型及貿易自由化環境下之租稅、民營化與貿易政策協調提供理論依據。

This paper develops an open-economy mixed oligopoly model with foreign export competition and an import tariff to examine how differences in public firm objectives affect the neutrality of profit taxation. By introducing a policy dummy variable, this study compares the traditional public firm model of Matsumura (1998) with the Public Social Enterprise (PSE) model of Chang et al. (2018) and examines the interactions among profit taxation, privatization policy, and import tariffs.
The results show that, under the traditional public firm model, the government can fully offset the output distortion caused by profit taxation by optimally adjusting the degree of privatization. Under the PSE model, although the profit tax directly affects the public firm's output decision and breaks the perfect substitutability between profit taxation and privatization policy, optimal privatization can still maintain aggregate output at the welfare-maximizing level. Thus, the neutrality of profit taxation remains robust across different public firm objectives.
Furthermore, the welfare effect of the import tariff is independent of the domestic profit tax rate when privatization policy is optimally adjusted. While a higher tariff can limit profit outflows to the foreign firm and increase tariff revenue, excessive protection may lead to production inefficiency under increasing marginal costs. Therefore, the optimal tariff should balance profit outflows and domestic production efficiency. These findings extend the literature on mixed oligopolies and the neutrality of profit taxation and provide theoretical implications for the coordination of tax, privatization, and trade policies.

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