According to outcrop profiles, drilling cores and well-loggings, the characteristic of sequence boundary and the type of boundary of Lower Ordovician strata are studied in the north part of Ordos Basin. The Lower Ordovician can be divided into 10 third-order sequences, which are SQ1 and SQ2 of Yeli Formation, SQ3 and SQ4 of Liangjiashan Formation,SQ5 to SQ10 of Member 1 to Member 6 of Majiagou Formation. Sequences SQ1 to SQ4are characteristic of shallowing-upward structure. Sequences OSQ5 to OSQ7 show regional transgression that is characteristic of aggradation under the subtidal environment. Sequence SQ8 reflects the maximum transgression with characteristics of volumes of condensed succession in this area. Due to affection of Caledonian event, crops of Sequence SQ9 to SQ10 exist only in the western and southeastern edges of this area, in which basinal deep-water sediments develop in the western edge and tidal flat sediments do in the southeaster edge.