Based on the observation of outcrop profiles, well cores and thin sections, combining with the analysis of geochemistry and petrophysics, the Lower Ordovician Majigou dolostones in Ordos Basin can be divided into dolomicrite, crystalline dolostone and dissolution dolostone. The dolomicrite is characterized by accompanying gypsum, high content of Sr and similar REE distribution pattern and δ13C distribution range to the micrite in place. The homogenization temperature of inclusions is approximate to that of earth′s surface, which implies penecontemporaneous dolomitization by fluid relating with surface-closing water or sea water. The crystalline dolostone is characterized by seldom accompanying gypsum, low content of Sr, and similar REE distribution pattern and δ13C distribution range to the micrite. The homogenization tempetrature of inclusions is high, which indicates that crystalline dolostone forms from burial dolomitisation by fluid relating with sea water. The dissolution dolostone are formed from the former two kinds of dolostones by atmospheric water. Intercrystalline pores and intercrystalline solution pores and dissolution pores are common reservoir space in Majiagou dolostone. Because of low porosity and permeability, the dolomicrite is hard to be reservoir rock while contributing to hight porosity and permeability, the crystalline dolostone can be good reservoir rock. The dissolution dolostone displays higher porosity and permeability so it can be excellent reservoir rock. It is suggested that the dolomite reservoirs being of exploration potential are in the central to eastern, southeastern and southern parts of Ordos Basin as well as Tianhuan area.