Customary law is a legal system that has been used by the community for a long time and it has been passed down from generation to generation. Bolit Mate Nawar Uman (BMNU) is a customary law used by indigenous people in West Kutai Regency, East Kalimantan. Even though there has been a shift in values and norms in society, BMNU customary law is still recognized and used by local communities in resolving legal disputes. Crime always appears where there is society, one of which is murder which, in this case, is studied according to the science of law and is associated with social life. The issues raised are how BMNU customs become living law when dealing with criminal acts of murder in West Kutai Regency and how BMNU customary sanctions can provide a value of justice for local indigenous people. This study uses a sociological juridical method with primary legal materials and secondary legal materials as well as a statutory approach and a case study approach. The results of the study show that the existence of BMNU customary law applies when the crime is committed within the territory of the customary territory and the victim of the crime of murder is a descendant of the Dayak tribe, West Kutai district. The application of customary sanctions for the criminal act of murder was tried by the method of deliberation between Dayak customary institutions and indigenous people in general. Keywords: customary law, Bolit Mate Nawar Uman, living law