This dissertation purports to propose a new theory of international constitutionalism based on the Kantian thesis of perpetual peace, the social contract theory, the German-Austrian tradition on State theory, and the post-WWII structure of the international legal order. The theory deals with the formation of an original contract and construction of a constitutional order in the international arena simultaneously and addresses three dimensions of public-private interactions therein—between international public authority and States, international public authority and civitas maxima, and sovereign States and territorial people. The theory provides a way of understanding State sovereignty by detaching sovereignty from State and reducing the main possible contexts that the notion sovereignty may arise. Notably, individual sovereignty does not dissolve in State sovereignty, and with the openness reserved in the domestic contract, the former constitutes an essential part of the international political life, which pertains to both the self-preservation of States and emancipation of civitas maxima. The broadest political imagination shall be a pluralist society consisting of individual sovereignty, State sovereignty and international sovereignty with their respective raison d’être. Meanwhile, the theory provides a way of reading morality, legitimacy, and justice. In international arena, legitimacy is no longer an intellectual interference to examine an already-existing public authority. Instead, it is the reason why international arrangements are made. This existential-functional reciprocal nature of legitimacy in international arena can only be understood in the process of legitimation. The gravity of the arrangements falls on building minimum solidarity and minimum normativity to form an international society and to construct an international sovereignty as the monopoly of legitimate use of force and the ultimate decision-making power for international public affairs. The key process lies in the transition of the external aspect of State sovereignty from a pure political being to a political-juristic being, and the creation of an international sovereignty evolving into a juristic-political being. With the mutual advancement of the public-private relationships on all three dimensions, humankinds may step by step get closer to a grand normative regime securing enduring peace and may maintain the capacity to restore such regime soon after a setback.