Abstract: In the past geologists had long held that the middle sector of the Greater Khingan Mountains in the Indosinian stage was an elevated terrestrial erosion area where the Triassic is usually absent. The Jilin Regional Survey Party discovered the Triassic when the party carried out 1:20000 regional geological surveys of the Wuchagou Sheet（L—51— Ⅶ） in 1978—1981.Then the Party established the Hadataolegai Formation （T₁h）whose age is temporarily assigned to Early Triassic.In 1986—1990, the Second Regional Survey Party of Inner Mongolia also found the Triassic Hadataolegai Formation when the party conducted 1:200000 regional surveys of the Junmachang Sheet（L-51-Ⅷ）and the 125-km Sheet（L-51-Ⅱ）in Solen. This formation overlies unconformably the Upper Permian Solon Formation （P₂s）.This paper presents a brief introduction to the distribution,sequence of strata,lithological features,geological age and geological significance of the Hadataolegai Formation.