Mercantile usages is one of the sources of law, recognized and practically applied in civil and economic law of many countries around the world, including Vietnam. In international law, the role of customary law is expressed in many areas such as international maritime law and international trade law ... In Vietnam, the Civil Procedure Code and the Commercial Law both stipulate that commercial practices are the basis for resolving commercial disputes. In fact, many commercial disputes have been resolved effectively through judges' reasonable application of local commercial practices, instead of applying legal documents. Based on reference to other countries in the world, Vietnamese commercial law stipulates that commercial practices are only applied when there are no provisions in the law and the parties have no agreement and no established habits between the parties. beside. This application must not violate the basic principles of Vietnamese law. Resolution No. 27-NQ/TW dated November 9, 2022 of the Political Bureau on Building and perfecting rule of law in the Socialist Republic of Vietnam in the new period emphasized the central task of "perfecting the legal system and the mechanism of law enforcement rigorously and consistently; ensuring the observance of the Constitution and the law". In the context that domestic commercial activities are dynamic and are the driving force for economic development, supplementing, updating, and improving the source of law is a mandatory requirement. The application of mercantile usage as one source of law has been established for a long time in Vietnamese legal practice. The paper contributes to clarifying the basis for applying mercantile usage more widely and popularly in domestic and international trade.