Abstract: By means of casting thin sections and scanning electron microscopy(SEM),high-pressure mercury injection,and etc,a comprehensive micro-pore structure study on Chang81 reservoir of Yanchang formation in Zhenbei area of Ordos basin has been carried out.The results show that the reservoir sandstone from Chang 81of Zhenbei area is primarily of medium-fine feldspathic sandstone,and secondly of feldspar lithic sandstone.Pore types in the study area are mainly composed of residual intergranular pore and secondary pore.The dissolution of feldspar,cuttings and carbonate cement is an important cause of the secondary porosity.Small and medium pores can provide main reservoir space for hydrocar-bon,while fine throat,throat and micro-throat can provide main seepage channel for reservoir.Based on morphology of capillary pressure curve and pore structure characteristics parameters,the pipe pressure curve of the study area can be divided into three categories.