Abstract: This paper reports new features of the dentition of Sinoconodon, an Early Jurassic mammal from the lower Lufeng Formation of Yunnan, China. Incisors and canines were replaced multiple times in Sinoconodon as in many non-mammalian cynodonts. New juvenile specimens of Sinoconodon have premolars which were replaced once before being permanently lost in the larger (thus older) specimens. Differentiation of premolars from molars and a single replacement of premolars are the derived features of modem mammals. The anterior molars (Ml— M2) were probably lost without replacement. The posterior molars (M3— MS) were replaced once. The skull experienced indeterminate growth while the teeth were being replaced in adult specimens. Sinoconodon is the sister taxon to all other mammals. The dental replacement of Sinoconodon could be interpreted as an intermediate stage in the character evolution from the primitive pattern of polyphyodont replacement seen in most cynodonts to the derived diphyodont replacement of mammals. From the polyphyodonty replacement and the substantial growth of the adult skulls of Sinoconodon, we infer that this taxon lacked the lactation and determinate growth typical of living mammals.