Abstract: A nearly complete skeleton of sauropod from the Upper J urassic of Zigong, Sichuan, is described in this paper. The new specimen (ZDM0126) is included in M. hochuanensis as its main similar features. But it has skull, teeth, the posterior caudals, pectoral girdle and fore limb which are absent in the holotype of M. hochuanensis, therefore, it complements some important features to this species, as follows: The features of the skull is generally similar to M. youngi. The skull is light and of Camarasaurus-type. The occipital condyle is hemispherical shaped. The basioccipital process is well developed. The paraoccipital process is well developed, its distal end is expanded and fan-shaped. The occipital keel of supreoccipital is well developed and triagle-shaped in dorsal view. The basipterygoid process of the basisphenoid is slender. The middle part of the parictal is very narrow, only 46mm in width. No pineal foramen is present. The frontal is thin and wide. The premaxillary is relatively short and thin, with a well developed nasal process. The maxillary is relatively long and large. The dentary is long and thin with relatively droopy anterior end. Teeth are small and spatulate, the crowns are relatively high and narrow. The medial ridges in the internal surface of the crown are well developed. Denticles are well developed on the anterior edge of teeth crown of the unworn teeth. The posterior caudals are fused with each other, their centra are not expended, the neural arch is remarkably expanded and the size of the neural canal and the height of the neural spines increased. In lateral view, the posterior caudals are cockscomb-shaped. The scapula is large and long, with a slender shaft. The coracoid is pointed and oval-shaped. The sternum is small and oval-shaped. The clavicle is short, with two processes in the proximal end. The humerus is short, almost 70 % the length of the femur. Both the proximal and distal ends are remarkably expanded, the deltoid crest is not well developed. The ulna is longer than the radius and 70 % the length of the humerus. The proximal end of the uala is relatively expanded with a prominent radiale-fossa. The radius is straight and flat. The femur is long and flat, its two ends are remarkably expanded. In the known species of Mamenchisaurus, cervical, dorsal, sacral, mid-anterior caudals are well preserved except for the posterior caudals. Therefore, the features of the posterior caudals are not known. The new materials show that the posterior caudals are fused with each other and are expanded and cockscombs-haped, which differ distinctly from the hammer-shaped tail of Shunosaurus and Omeisaurus. We suggest that its posterior caudals have two main functions: it serves not only as a defensive weapon, but also as an acute sense organ.