Abstract: In this paper, the authors investigated characteristics of microscopic pore structure in low permeability sandstone reservoir for providing the basis for further exploration and development. By means of physical property analysis, SEM, casting slice, high pressure Hg injection, and rate-controlled Hg penetration, the authors selected low permeability sandstone samples from Chang 9 of Yanchang Formation in Zhijing-Ansai area of Ordos Basin and made analysis. The information of reservoir pores, types and the morphology of throat and its degree of connectivity was obtained, and the quantity factors about the size of pore and throat, sorting and connectivity were determined. The results show that the pores mainly consist of residual intergranular pores, solution pores and micro-pores. The pore combination is divided into four types which reflect the diversity of pore structure. Pore structure of the samples is characterized by large pore and fine throat. The development of fine throat is the main reason responsible for ultra-low permeability.The diversity and heterogeneity of the reservoir pore structure is the main cause of the reservoir heterogeneity.