Abstract: The Zongbayin Sag is subordinate to the depressions in the middle of East Gobi Basin in Mongolia. It is a Meso-Cenozoic sag formed on the Paleozoic fold basement, of which basins are superimposed and depression-faulted of inversion type. The Early Cretaceous rift basins take the majority and they experience four stages of development, i.e., faulted subsidence, faulted inversion, depression subsidence and depression inversion. At the stage of depression inversion at the end of Early Cretaceous, Zongbayin early synsedimentary slope was transformed into thrusting with broken nose, because of the sinistral strike-slip of Zongbayin fault along the arc-shaped cross-section. So the "framework of three bulges and two depressions" of the Zongbayin Sag takes shape now. Same as other sags in the Erlian Basin, the Zongbayin Sag is part of rift basin groups in Northeast Asia. However, it is located in the transitional region of the extensional structures in the east and the compression structures in the west, of which the geotectonic environment is more like the top of the Mongolia arc structures. So there is similarity but with difference between the Zongbayin Sag and other oil-rich sags in the basin. They have similar history of structural development, complex inner structures, NE and NNE fracture system. But they have different basin types and development periods, different sag structure and framework, different causes and formation time of local structures

Key words: East Gobi Basin, Zongbayin Sag, Erlian Basin, history of structural development, structural characteristics, fracture system