Abstract: The fourth and fifth members of the Yanchang Formation (Chang4+5) in Zhenbei area of the Ordos Basin are low-permeability reservoirs of diagenetic origin. Conventional methods such as thin section analysis, scanning electron microscopy and physical properties determination failed to analyze the microscopic pore structure of these reservoirs. We used conventional methods and applied constant rate mercury penetration to study the controlling factors to quantitatively determine microscopic pore structure. The microscopic pore structure of the Chang4+5 reservoirs was mainly impacted by diagenetic effects. When mechanical compaction and carbonate cementation were weaker and feldspar dissolution was more intense, intergranular pores and dissolution pores would be more developed, resulting in better connectivity and higher permeability. Microscopic pore structure and permeability were mainly controlled by throat radius. Generally, if average throat radius was >0.12μm, permeability would be higher, the smaller the pore/throat radius ratio, the better the microscopic pore structure, the greater the reservoir quality index (RQI), and the higher liquid oil yield, especially when the average throat radius was <1.26μm. Throats controlled the quality of the microscopic pore structure of low permeability reservoirs.