Abstract: Four Eocene mammals were reported from the Bayan Ulan locality, Nei Mongol, China, including a new rodent species— Mergenomys neimongolensis. The new species differs from the type species of the genus, M orientalis, in being larger and having the ectolophid and hypoconulid more labially positioned. Based on the fossils and stratigraphic correlations, the beds that were previously considered to be "Irdin Manha Fomation" in the Bayan Ulan section are actually two rock units: the Irdin Manha formaüon below and the Shara Murun formation above. The new materials come from the late Middle Eocene Shara Murun formation.