Abstract: During the regional geological survey, the Gegenaobao Formation, which had been regarded as the Lower Permian Dashizhai Formation, was discovered in Zhalantun of Inner Mongolia. The strata are NE striking, Late Carboniferous-Early Permian in age, with rock association of andesitic lava, intermediate pyroclastics and normal clastics, containing animal and plant fossils, to be a volcanic-sedimentary rock series of paralic facies. This formation is firstly found in Zhalantun area. The discovery is importantly significant for basic geological study, such as tectonics and stratigraphic division.