Abstract: The mass mortality of birds in Early Cretaceous in Sihetun area, Beipiao, Liaoning Province was once considered to be caused essentially by the toxic gases of SO₂, H₂S, PbBr₂, PbBrCl, Pb (OH)Br, (PbO)₂PbBr₂ and Hg from volcanic eruption.By study on the physiological and habitual characteristics of birds, the evolution and eruption of volcanoes in the region, as well as the producing conditions, physical and chemical characters and toxicity of CO and the hydrides of C, As, Sb, Bi and Pb, the authors conclude that the lethal gases are the hydrides and CO released in the eve of eruption.The birds were buried by the volcanic ash from the afterwards eruption, and then evolved to fossils.