During the Early Ordovician, a suit of carbonate-evaporites had deposited in the east part of the Ordos, which is now known as the Majiagou Formation and up to approximately 1000 meters in thickness.The rocks of the Majiagou Formation can be classified into 18 types of microfacies according to the mineral composition, microscopic structure and fossils. These microfacies types can be grouped into 6 series genetically, which are open sea-related microfacies series, grain-shoal-related microfacies series, secondary crystalline granular textural microfacies series, reef (mound)-related microfacies series, cryptalgal mat-related microfacies series and restricted-intense evaporation-related microfacies series. The microfacies in each series are similar in genesis and environments of formation, and their shifts are relatively continous.The results of microfacies analysis indicate that: (1) most of the limestones of the Majiagou Formation formed in open sea shelf environments and tidal flat environments, and part of them was related to the development of reefs; (2) the evaporites, such as anhydrites and halite, had formed under the condition of evaporation, because of the saline depression's periodic isolation from the open sea; (3) the dolomites have 2 genetic types: one related to sabkha environments and another related to diagenetic environments during buring.