Abstract: There are different views on the geological ages and scales of the unconformities within the Ordovician strata in Tarim basin. The Yingshan formation is the key hydrocarbon exploration target in the Ordovician strata of Tarim basin, but up to now, researchers still consider that the Yingshan formation should be classified into the Middle-Lower Ordovician, so the unconformity within the Ordovician strata should be the unconformity between the Middle-Lower Ordovician and the overlying strata in Tarim basin. Based on the study of conodont fossils, the author considers that the largest unconformity within the Ordovician strata in Tarim basin initiated from the late of Early Ordovician and is widely distributed across the basin, which exists between the Yingshan formation and the overlying strata in platform area and is found within Xiangguletage formation in the profile of Kuluketage outcrop in the slope area