The Urtu fauna, Ulantatal fauna and Keke'amu fauna were discovered indifferent Tertiary horizons in the Nao'ertao area, Alxa Left Banner, Inner Mongo-lia. Based on the stratigraphical and paleontological data, a new scheme of stra-tigraphical division of the Tertiary has been proposed, in which the upper Mio-cene Huhehaolai Formation is redefined and the lower Miocene Urtu Formation,middle Oligocene Ulantatal Formation and lower Oligocene Yuzuitao Formation areestablished. Gomphotherium sp. and Megacricetodon sp. in the Urtu fauna that holddefinite horizons are the oldest and most primitive forms in Eurasia, and thelatter might be the earliest form on the earth. The Keke'amu fauna is the earli-est one of the middle Oligocene known so far in Asia.