Abstract: An incomplete theropod skeleton including partial skull, mandibles, ilia was collected from the Early Cretaceous Jiufotang Formation of Kazuo County, western Liaoning Province. It can be estimated that this theropod may reach 9~10 meters in total length. It should be classified within Tyrannosauroidea by its tall premaxillary body, median vertical ridge on the external surface of ilium. It differs from the Late Cretaceous tyrannosaurids by much large external naris, slightly concave anterior portion of maxillary dorsal margin, maxillary fenestra reaching past the rostral margin of antorbital fossa but keeping apart from the ventral margin of antorbital fossa. Thus, Sinotyrannus kazuoensis gen. et sp. nov. is erected, perhaps representing the earliest member of Tyrannosauridae. This is the largest known pre-Late Cretaceous tyrannosauroid, and also the largest theropod from the Jehol Biota of western Liaoning and the adjacent regions. The discovery of this taxon not only suggests that East Asia should be one of the most important evolutionary centers of tyrannosauroids, but also provides the significant evidences for studying the origin of Tyrannosauridae, and the composition and ecosystem of the Jehol Biota.