行政契約法長期爲我國法制所忽略，固然與其法律救濟途徑之結構有直接密切之關係；但更深遠地說，實是我國行政法深受德國法制影響所致。 德、法兩國行政契約法之主要差異，粗略而言，即在於德國法上之行政契約仍然強調當事人間“地位平等”的私法契約觀，而法國之行政契約法自始即以行政機關基於公共利益之實現而享有優勢地位，作爲行政契約不同於私法契約的主要特徵。基此，德國行政契約法側重人民權利之保護面向，而法國法則著重行政契約的工具性效能。 我國行政程序法之制定，原以德國行政程序法爲主要範本；其中有關行政契約的部分，尤其與德國行政程序法之相關規定十分接近。惟學者研擬本法草案之際，特別將法國行政契約法上強調行政權優勢的精神納入考量；進而將此特色植入本法相關條文之中。其中第一百四十六條有關行政機關單方變更權的規定，即同時兼具法、德兩國法制之特色：一方面強調行政權所代表的公益性，另一方面亦不忘締約人民的權利保護。本文即以在常態情況下，行政機關所享有之單方變更權爲出發點，一方面探究單方變更權在行政契約法上的制度設計與其理論基礎；另一方面則討論其實際運用的可能領域及其適用原則。

Administrative contract law, in our legal system, is neglected for a long time, due to the special system of its contentious proceedings undoubtedly. However, the real reason is that our administrative contract law is deeply influenced by the German contract law. The main difference between the German and the French administrative contract law, roughly speaking, is the fact that the German administrative contract still puts great emphasis on equality between two parties; and yet the French public law gives the administrative authority more privileges because of public interest. Therefore, German law places importance on protection of the people's rights; while the French law emphasis on the instrumental efficiency of the administrative contract. The Administrative Procedure Act of ROC is very similar to the Germany's Administrative Procedure Act. But the scholars, who drew up the draft, put the spirit of French administrative contract law into our Administrative Procedure Act. Article 146, about the power of unilateral modification, is united the characteristic of German and French law. On one hand, it emphasizes on public interest that the administrative authority represents for; on the other hand, it never forgets the protection of people's rights. This article sets out from the power of unilateral modification in the normal condition. We study the institutional design and theoretical foundation of the right of unilateral modification; and discuss the field of application and the principle of putting it into practice.