Abstract: Archaeopteryx has been recognized as the earliest known bird in the world for more than 140 years, and it is well documented from seven skeletons with feather impressions and an isolated feather. Archaeopteryx shows a combination of diapsid ("reptile-like") plesiomorphies and the derived ("bird-like") features. It provides the most important evidence for the hypothesis that birds originated from a dinosaurian ancestor. Since 1990s, the rich assemblage of taxonomically diverse and numerically abundant feathered theropods and primitive birds have been found from the Early Cretaceous Yixian Formation of western Liaoning, China. These findings further strengthen the hypothesis that birds were derived from small theropod dinosaurs. However, no avialian birds were found from China at that time. Until 2002, two avialian birds, Shenzhouraptor sinensis and Jixiangornis orientalis, were reported from the Early Cretaceous beds in western Liaoning, China. Here we describe a new well-preserved avialian bird from northern Hebei, China. Our phylogenetic analyses have placed it at the root of the avialian family tree. It sheds new light on the phylogenetic relationship between non-avian theropods and avian theropods (birds) and on the definition of birds.