Abstract: Trace fossils from the Middle Ordovician Lashizhong Formation in Zhuozishan, the northwest margin of Ordos Basin are studied systematically. The Paleodictyon assemblage, including Paleodictyon of majus, P. sp., Megagrapton irregulare, Neonereites biserialia, Planolites vulgaris, Chondrites filiformis, Ch．furcatus and Zoophycos of brianteus, has been recognized for the first time in the studied area．The trace fossil assemblage is characterized by the abundant and spotted trace fossils and the wide spread occurrence of Paleodictyon．The classificatory characteristic of the trace fossils, such as morphology, behavior, preservation, etc., indicate that the Paleodictyon assemblage corresponds to the Nereites ichnofacies representing depositional environment of far-turbidity current in deep sea．During the late period of sedimentation．however, the water depth became shallower as the environment of turbidity current in continental slope rising. The characteristics of sedimentary sequence, sedimentary structure and lithology in the Lashizhong Formation indicate a typical turbidite facies deposits, especially far-turbites facias. According to the palaeocurrent measurement, it is inferred that the directional change of the gravity currents roughly reprepsents the trends of decrease of extensional faulting and of uplift in the Helan aulacogen during the Middle Ordovician period ．