Abstract: There is a set of metamorphic rocks in Ondor Sum region, Nei Mongol, which was named as Ondor Sum Group and its age was considered as Paleozoic before. Tow major assemblages of the Group can be recognized. One is outcropping at Wulangou and consists of metamorphic pillow basalt, clastic rocks, silicalite and other epimetamorphic rocks. The other is outcropping at Deyanqimiao and consists of plagioamphibole, plagiogneiss and other hypometamorphic rocks. The assemblages are different from each other in metamorphism, deformation and petrologic chemistry as we1l as the tectonic settings. The results of this study demonstrated that the volcano-elastic assemblage was formed in back-arc basin, and the hypometamorphic assemblage was the underplatting product during extention in the region. Recognization of the DNO assemblages has an important significance for the tectonic evolution of the northern margin of North China Plate. According to the recent isotopic chronological data, the ages of two assemblages are all Neo-Proterozoic. The Sm-Nd isotopic age of the volcano-elastic assemblage is 807～961Ma, and the hypometamorphic assemblage is about 638Ma. The rifting began in the Meso-Proterozoic at northern margin of North Chins Plate. At the beginning of the Neo-Proterozoic, there was a series of island arcs separated from the North China Plate and back-are basin occurred at the same time. The volcano—elastic assemblage were formed in this period. Another extensional cycle began at the end of the Neo-Proterozoic. The hypometamorphic assemblages were formed in the magmatic passive margin during underplating. Since the major assemblages of the Ondor Sum Group are the products of different stages in tectonic evolution of North China Plate in Neo-Proterozoic, the tectonic evolution in the Paleozoic must be reconsidered.