Abstract: The marine carbonate rocks were discovered for the first time in the Xainza region during the regional geological surveying of the 1:250000 Toiba Sheet in Xizang. These strata contain abundant Boreal Actinoceras faunal fossils such as Armenoceras sp., Wutinoceras sp., Discoactinoceras multiplexum Kobayashi, Adaomsoceras sp., Sactoceras sp., Ordosoceras sp., Oncoceratida, Ormoceratidae,?Curtoceras sp., Ormoceras sp., Liulinoceras sp., Deiroceras cf. globsom Zholt et Shen, brachiopods, conodonts, bryozoans and crinoids. The characteristics of lithologic associations and faunas found here appear to be in conflict with the existing strata in the study area, and the geological ages of the strata are obviously earlier than the other Palaeozoic strata in this area. This is why the Lower Ordovician Lhasai Formation is suggested for the Xainza region, northern Xizang.