A set of variegated transition beds of fine-clastic rocks was continuously between coal-bearing beds of the Upper Triassic Shezi Formation and red beds of the Lower Jurassic Lufeng Formation (formerly Lower Lufeng Formation) in central Yunnan and its adjacent areas. It is persistent and was recognized as a part of the Shezi Formation and assigned to the Upper Triassic. In this paper, a new formation, the Yubacun Formation, is erected for the transition beds according to the lithological, biological and magnetostratigraphic characteristics. The new unit is identified to be Jurassic in age and its lower boundary is the lower boundary of the Jurassic System.