Abstract: Petrological characteristics of bedded radiolarian cherts and different types of volcanic tuffs have been studied in the Ordovician of Weibei, Shaanxi. Distal eruptive materials supplied SiO 2. Radiolarians thrived largely in this condition, and gradually dead bodies of radiolarians were accumulated on sea bottom and turned finally into bedded radiolarian cherts with sediments. It is inferred further that the Middle Upper Ordovician were deposited in a pelagic or semipelagic deep water environment with a greatly rising sea level. Volcanic tuff is the product of island arc volcanic eruption. This area was once the north margin of a back arc basin which belonged to an active continental margin of the North China plate.