Abstract: In modern international law, treaties are an important carrier to create customary international law.There are two situations to form customary international law by way of a treaty: One is certain treaty rules' becoming customary law, the other is a treaty as a whole becoming customary law.The former has three forms: (1) codifies existing customary rules; (2) crystallizes emergent customary rules; (3) initiates the progressive development of customary rules.To create customary law by these ways, the requirements of a treaty's law-creating quality and universality, general practice and opinion juris must be satisfied.In addition to the above four requirements, the latter must meet two conditions, that is, there are a great number of contracting states whose interests have greatly influenced by the treaty and the treaty cannot be reserved, thereby adopting the form of a package agreement.All above four forms have been applied in UNCOLS 1982.