Abstract: A new genus and species of docodontid (Docodonta, Mammalia),Acuodulodon sunae, represented by a partial left lower jaw and dentition, is described. It is from the upper part of the Shishugou Formation in the Wucaiwan area of the Junggar Basin in northeastern Xinjiang, China, with an estimated age of 159~161 Ma (Oxfordian, early Late Jurassic). The new mammal is typical of docodonts in having a cusp b in front of cusp a, a cusp c distolingual to cusp a and a cusp g mesiolingual to cusp a on lower molariforms. Differing from other docodonts, it has no cusp e or crest b–e developed on lower molariforms. Unique among docodonts, cusps a and c of the new animal maintained their sharpness while cusp g and crest b–g wore away fast, indicating that both shearing and crushing/grinding occurred in the chewing cycle and probably last for most of the life span of the animal. Phylogenetic analysis of a data matrix with 24 lower molariform characters across 15 taxa recovers a monophyletic Docodonta, which has distinct diagnostic characters in lower molariforms. Within docodonts, Docodon andBorealestes are successively basal to other docodonts; Acuodulodon andItatodon + (Simpsonodon, Castorocauda + (Tegotherium + Sibirotherium)) form a monophyletic clade. Tegotheriid genera are nested within Docodontidae, but a monophyletic tegotheriid clade composed of Tegotherium,Sibirotherium, Itatodon, and Tashkumyrodon is not recovered. The dentary ofAcuodulodon is typical of docodonts. It has a shallow postdentary trough and a wide and sharp medial ridge, both of which do not extend onto the medial side of the condylar peduncle, indicating looser contact between postdentary bones and the dentary than in morganucodontids, a more derived condition in the evolution toward the definitive mammalian middle ear.