Abstract: During the 1:50000 regional geological survey in Honggor area on the north of Erenhot, Inner Mongolia, near the border between China and Mongolia, the Ordovician Wubinaobao formation is redefined on the basis of rock association, paleontological characteristics, depositional environment, etc. The revised Wubinaobao formation includes a set of layers of slate and siltstone with intercalations of thin limestone lenticules, containing fossils of Trilobita, Brachiopoda, etc. The strata are characterized by neritic shelf sedimentation. This redefinition is significant for the oreprospecting in this area from now on.