Abstract From the North Sea Continental Shelf Cases in 1969 to the recent maritime delimitation dispute between Peru and Chile, the International Court of Justice (hereinafter ICJ or the Court), on the one hand, rejects the mandatory nature of equidistance method, result in a non-priority statue of it in delimitation methods, and meanwhile endorses the settlement of maritime disputes on the basis of equitable principles; on the other hand, the Court is convenient to apply equidistance method as a starting point in maritime delimitation, which can be founded in lots of Judgments of the ICJ. So-called ―two faces‖ in this thesis means that the inconsistent attitude of the ICJ concerning the equidistance in the maritime delimitation. The purpose of this thesis is to find the reasons why the ICJ has played a paradoxical role on application of equidistance in maritime delimitation. It involves probing for deeply rooted questions, what is the foundation of the law of international maritime delimitation, and what is the criterion of maritime delimitation. In order to answer these questions, besides the brief introduction at first chapters, this thesis consists of three main chapters. Chapter II and III traces respectively the historical development of equidistance principle, which is crystallized into equidistance/special circumstances rules, and the formation and evolvement of equitable principles. In these two contexts, the respective contributions of treaty law and case law will be analyzed for the emergence and promotion of the two principles. Chapter IV focuses on the core issues of maritime delimitation. For this purpose, firstly, it is important to examine what is maritime delimitation; what is the criterion of maritime delimitation; and where this kind of criterion derive from. Based on these rationales of international maritime delimitation, the reason of ―two faces‖ is not confusing any more. Respectively, maritime zoning is based on the entitlement and follows the criterion of distance, namely ―the land dominates the sea‖ or proximity, not the natural prolongation. Thus maritime delimitation is delimited by the criterion of equidistance on the basis of overlapped entitlement. That‘s the reasons why equidistance/special circumstances rule, which stems from the principle of maritime zoning, is always preferred applied by the ICJ in maritime delimitation. On the other hand, it is the principle of equity, rather than the undefined notion of equitable principles, that forms part of the normative framework of maritime delimitation. The role of equity, as a negative condition, is to ensure the balance of rights and interests attained for the overlapping of entitlements and leave a room for accommodating unforeseeable case. This should be the significance of insisting on the principle of equity by the ICJ in delimiting maritime boundaries. After all the mentioned issues have been resolved, a general conclusion of the whole thesis is arranged in the last chapter. KEYWORDS Equidistance Method; Equitable Principle; Maritime Delimitation.