There is a long-standing debate on the weight or preference given to different sources of international law in jurisprudence. This article aims to discuss the interplay of three pairs of sources of international law; namely between old treaties and new treaties, treaties and customs, and existing treaties and emerging treaties in the context of three regions which are featured with typical maritime related issues. In the Arctic region, the 1925 Svalbard Treaty and the 1982 United Nations Convention on the Law of the Sea (UNCLOS) become the sources of conflicts or different legal positions between Norway and some other States who are parties to both treaties. In the South China Sea, the major legal issue, among many other important ones such as island regime, is the relationship between UNCLOS as a treaty law and historic rights as a customary international law. The Antarctic Treaty System (ATS) may be influenced by the new instrumental arrangements made by Biodiversity Beyond National Jurisdiction (bbnj) negotiation, reflecting the importance of ensuring the emerging treaty will not interrupt the jurisdiction of established treaties.