Abstract: A specimen collected from the Early Cretaceous lowest part of Yixian Formation of Liaoning, northeastern China, represents a new genus and species of dromaeosaurid theropod. It comprises a fragmentary maxilla with some teeth, a few caudals, almost complete fore limbs, and partial hind limbs and is here named Graciliraptor lujiatunensis gen. et sp. nov. Distinctive characteristics of the new species include a laminal structure connecting the postzygapophyses of middle caudals, middle caudals extremely long and slender, small manual ungual I, proximal end of metacarpal III strongly expanded, proximal tibiotarsus shaft rectangular in cross section, astragalar medial condyle significantly expanded posteriorly, metatarsal II distally much wider than the other metatarsals and long and slender pedal phalanx III-1. Being the earliest definitive dromaeosaurid species known to date, G. lujiatunensis provides new information important for understanding the early evolution of the group. On one hand, G. lujiatunensis displays a few features similar to those of basal birds, such as caudals significantly elongated, semilunate carpal small and primarily contacting metacarpal II, and manual digit I short, providing further evidence for a close relationship between the Dromaeosauridae and the Aves; on the other hand, it is similar to troodontids in some features on the caudals. The discovery of G. lujiatunensis also indicates a high diversity of the Dromaeosauridae in the Early Cretaceous Jehol Biota. Combined with other lines of evidence, it is inferred that the Dromaeosauridae rapidly diversified taxonomically but remained relatively stable morphologically in the early evolution of the group. Key words W