Abstract: Based on the progress of the recent study on marine strata and correlation of terrestrial with marine strata in the world, and on the new dating of the Eocene—Oligocene boundary (34 Ma), the Mongolian Hsanda Gol Formation should be of Early Oligocene in age rather than Middle Oligocene thought in the past The Ulantatal Fauna of China has been widely used since its discovery, and has been assigned as Chinese Middle Oligocene Land Mammal Age (Tong et al., 1995). This fauna, however, shows great similarities with Hsanda Gol Fauna. The former has 1 7 of 21 genera studied and 14 of 33 species in common with the latter, which indicates that the Ulantatal Fauna is of Early Oligocene instead of Middle Oligocene in age. A new division of the Chinese Oligocene is also proposed in the present paper.