The UN Convention on the Law of the Sea (UNCLOS), explicitly or implicitly, leaves significant room for the interpretation and application of customary rules on certain matters under the international law of the sea. This paper explores the recent developments of international practice dealing with the relationship between UNCLOS and customary law. It attempts to evaluate the jurisprudence reflected by international cases and state practice concerning the relationship between conventional law and customary law in general and suggests that certain subject-matters in the law of the sea concerning maritime rights, including historic-related rights are principally governed by general international law including customary law.