The Chang 4+5 oil reservoir set which is composed of Chang 4+52 and Chang 4+51 sublayers in Nanliang area in Ordos Basin is an important oil reservoir. Based on petrological and physical features, this paper studied the characteristics of Chang 4+5 oil reservoir set and evaluated reservoir properties. The result shows that the reservoir sandstone is compose of arkose and lithic arkose; the pore types are mainly intergranular pores and dissolved pores; the main throat type is fine to micro-fine throat and the reservoir displays high displacement pressure. It indicates that Chang 4+5 oil reservoir set belongs to ultra-low permeability reservoir. Sedimentation and diagenesis are the main factors of ultra-low permeability. Sedimentation controlled the distribution of reservoir. Compaction and cementation decreased the primary pores, while chlorite-cementation, dissolution and hydrocarbon emplacement play constructive role of the porosity building. Based on lithology and physical properties, the reservoir rocks can be divided into two types. The favorable reservoir should be in the subsea distributary channel sand bodies controlled by sedimentation.