The bone phallus is an important characteristic for species identification in Microdalyellia.In this paper,the shape and development of the phallus were observed in both M.sinensis and M.hunanensis.The results have shown that the branch characteristic in the distal branch of the phallus comes into being in early development and remains all the life;the complete ossification of the phallus occurs at 12 d in both M.sinensis and M.hunanensis;and the stem of the phallus becomes longer and bigger gradually along with individual's growth.We therefore suggest that the branch characteristics in the distal branch of the phallus are reliable taxonomy criteria in Microdalyellia.