Due to the special structures and the depositional setting of Ordos Basin, a large scale low-permeability reservoir is formed there. Based on the studies of scanning electron microscope, casting sheet image, core observation, physical property analysis and core analysis testing, the microscopic features of the ultra low permeability reservoir in Ordos Basin X area Chang6₃ are analyzed . It is shown that the ultra low permeability reservoir is controlled by deposition and diagenesis. The area has few slender rock, and is mainly covered with sandstone, and high interstitial materials. Remaining primary intergranular pores and dissolution pores serve chiefly as the storage space. As far as its physical properties are concerned, the reservoir belongs to a low porosity-extra low porosity-ultra low porosity-ultra permeability reservoir; the reservoir is mainly in the fine pore narrow throat mode. The reservoir microscopic features are controlled by macro geological factors. The deposition decides the reservoir's lithology, interstitial content and sizes of primary pores. The compaction and the cementation of diagenesis destroy primary pores, but the denudation provides a wide range of secondary pores, which improve the storage space of the research area.