This article examines the application of John Stuart Mill's formal equality theory to inheritance rights of adopted children within Indonesian legal frameworks, specifically the Burgerlijk Wetboek (BW) and Compilation of Islamic Law (KHI). Using a normative juridical approach with secondary data sources including statutory provisions, judicial decisions, and philosophical texts, this research addresses whether differential treatment of adopted versus biological children comports with Mill's formal equality principle. Findings reveal significant normative tensions: both legal regimes restrict adopted children's inheritance rights based primarily on biological lineage, which Mill's theory would deem insufficient justification for unequal treatment. Under the BW, adopted children possess limited rights unless formally adopted through prescribed mechanisms, while the KHI denies adopted children rights under the faraidh system, permitting only wasiat wajibah up to one-third of estates. Judicial interpretation shows modest accommodation through mandatory bequest provisions, yet falls short of full equality. This research concludes that current Indonesian inheritance frameworks contain structural inequalities conflicting with Mill's formal equality principles, necessitating legal reform to eliminate discriminatory provisions based solely on biological status. The article contributes to discourse on harmonizing civil law, Islamic law, and liberal egalitarian principles within pluralistic legal systems.