This article examines the historical roots, social implications, and inherent paradoxes of jus sanguinis-a legal principle that uses blood lineage to defines one's nationality-during the years of state succession in twentieth-century China. The study begins with a discussion of why the Qing dynasty established the first Chinese nationality law and chose jus sanguinis over jus soli at a time when anti-Manchu ethno-nationalism was on the rise. Qing lawmakers imported and invoked jus sanguinis not to define an exclusive or homogeneous nationhood, but to maintain individuals' perpetual allegiance to the state. In periods of state succession, legal cases involving nationality law reveal the paradoxes inherent in defining Chinese national membership and loyalty by bloodline, though the principle itself is practiced as if objective. Today, jus sanguinis still defines Chinese nationality and requires nationals' loyalty to China, with unintended, problematical consequences for people from border-crossing communities and historical borderlands.