There excited an unconformity between the Lower—Middle Ordovician Yingshan Formation and its overlying strata in Tazhong—Bachu area of the Tarim basin, Northwest China. Systematic and detailed studies on conodont fauna indicate that the strata under the unconformity have various ages in different areas, and the strata over the unconformity is nearly the same horizon, namely the Lianglitag Formation of Upper Ordovician. In Tazhong area, the Lianglitag Formation is directly over the Member 3 of the Yingshan Formation, which indicates that the absent strata include the Members 1 and 2 of the Yingshan Formation, the Yijianfang Formation, the Tumuxiuk Formation; and partially the lowermost of the Lianglitag Formation. This stratigraphical lacuna equals to 11 conodont biostratigraphical zones at least, which is inferred to be an interval of about 17~20 Ma long. In Bachu area, west of Tazhong area, the Lianglitag Formation is underlain by the Member 2 or even Member 1 of the Yingshan Formation, which implicates that the strata lost in the unconformity is less than that of Tazhong area. The lost strata equals to at least 8 conodont zones, indicating a stratigraphical lacuna of 14 Ma. This unconformity is important for the development of the karst reservoirs in the Yingshan Formation and is significant for the oil—gas exploration in the Ordovician of the Tarim basin.