This study examines the comparison between Law Number 1 of 1974 on Marriage and Presidential Instruction Number 1 of 1991 on the Compilation of Islamic Law (KHI) in the context of family law regulation in Indonesia, particularly for Muslims. Using a normative juridical and comparative approach, the research analyzes the similarities and differences between the two legal instruments from philosophical, juridical, and sociological perspectives. Law No. 1 of 1974 is national in scope and applies to all Indonesian citizens regardless of religion, whereas the KHI is a codification of Islamic law specifically applicable to Muslims, enacted through a Presidential Instruction. Fundamental differences lie in legal principles, provisions on polygamy, marriage guardians (wali), and divorce. The findings highlight the need for legal harmonization between the universal norms of national law and Islamic values embedded in the KHI to ensure legal certainty that is just and responsive to the diverse social fabric of Indonesian society.