Abstract: This paper mainly deals with the stratigraphical and tectonic significance of several groups of isotopic ages obtained from the Boin Sum Group rocks. There have been a lot of disputes about the original Boin Sum Group. In this paper the group is divided into two parts: （1） the Boin Sum Group: middle-upper Proterozoic in age, with a zircon age of 1525 to 620 Ma; （2） The Xuniwusu Group: Silurian in age, containing coral fossils, with a Rb-Sr isochron age of 427.5±17.5 Ma. It has been determined preliminarily that the early Paleozoic paleo-island arc in the vicinity of Boiu Sum in the middle part of Inner Mongolia is a continental-magin island arc. Its volcanic chain occurs as islands striking nearly E-W, and its back-arc area is a shallow sea.