Abstract: A new species of sinamiid fish, Sinamia liaoningensis, is described from the Barremian-Aptian Yixian and Jiufotang formations of western Liaoning, China and the morphology of the sinamiid fishes is reviewed. The new materials reveal three synapomorphies of the Sinamiidae proposed by Grande and Bemis (1998): the presence of only a single median parietal; three pairs of extrascapular bones; and short dermopterotic about equal in length to parietal length. The new species was attributed to Sinamia because it has three synapomorphies of the genus: relative small postinfraorbitals, a long dorsal fin and rhombic scales. The new species is a big podgy fish and it differs from all other species of the genus in the shape of the rostral, nasal, preopercle, the size and shape of infraorbitals, more subinfraorbitals (3), less dorsal fin rays (18), more caudal fin rays (16), more scale rows (32) between the origin of the anal fin and the dorsal margin of the body, and lack of serrated scales. The anterior half of the caudal fin rays of S. liaoningensis sp. nov. has remains of thin fibrous actinotrichia.