Causa finalis is the last one of the four causes provided by Alberico Gentili in his main work De iure belli libri tres to construct a just war. According to his own definition, it means "the rights of victor and vanquished, the result of victory, and the methods of ending a war". His analyses of causa finalis intensively appear in the third book. The first chapter is a short introduction declaring that the end of war for which all ought to strive is peace. And the rest of the book is about how to establish a permanent peace. The first half (chaps. 2-13) deals with the post-war process led by the victor, and the second half (chaps. 14-24) discusses the matters related to the conclusion of peace treaties. With respect to the post-war process, Gentili thinks that the victor should behave generously with self-control, otherwise the peace will be short-lived. With respect to peace treaties, Gentili calls for a clear distinction between agreements of sovereigns representing the state and those of sovereigns as a private person. Considering the result of former research on causa efficiens, causa materialis and causa formalis, we can grasp the entire structure of De iure belli libri tres. Through this work, Gentili intended not only to describe his standards for a just war, but also to compose a new legal system among early modern national states, that is, international law. It contains both peacetime and wartime criteria and will contribute to the establishment of permanent and ultimate peace.