Two species of "Sinornithosaurus" have been described. "S. millenii" ("millennium Chinese bird-lizard") is the type species, described in 1999. A second species, "S. haoiana" ("Hao's Chinese bird-lizard") was described by Liu "et al." in 2004 based on a new specimen, D2140, which differed from "S. millenii" in features of the skull and hips. However, according to Turner, Makovicky and Norell (2012) the supposed distinguishing features of "S. haoiana" "are either present in "Sinornithosaurus millenii" or variable among the number of "Sinornithosaurus" specimens"; the authors considered "S. haoiana" to be a junior synonym of "S. millenii".