ABSTRACTThe implementation of customary inheritance law in Indonesia is very dependent on the defeat applied in indigenous peoples and determines one's position in adat. The Mandailing indigenous community with a patrilineal kinship system applies a system of inheritance only to boys and is only a gift for women or inheritance if a will is available beforehand. The study uses empirical judicial and analytical descriptive methods with observational research methods and interviews with the Chairperson of North Sumatra Muslim Pardomuan (PMSU) and also the main respondents to reach general conclusions. Responding to the Mandailing community per supervision of Palangka Raya City which is incorporated in the Palangkaraya City PMSU applies Islamic inheritance law so that the patrilineal system does not apply to Mandailing indigenous people per supervision and assets are not given only to boys, but daughters of Islamic inheritance, although including the fact that one respondent's family gave up the inheritance to be managed by the sons of seven siblings and used it for the benefit of all, and the delegation began with the distribution of inheritance by Islamic law. Finally, the implementation of customary inheritance law in Mandailing customary tribe per monitoring is still based on Islamic inheritance law. The influence of Islam inherent in the Mandailing indigenous people per the main factor determining their decision in taking Islamic rules in their inheritance system. Keyword: Inheritance Law and Mandailing Customary