Abstract: The Chang 4 + 5 reservoirs in the Fengdikeng-Hongde region, Ordos Basin are dominantly composed of lithic feldspathic and feldspathic sandstones. The present paper deals, on the basis of cores, thin sections, SEM and clay minerals, with the diagenesis of the reservoir sandstones from the Chang 4 + 5 member of the Yanchang Formation and its affects on the reservoir physical properties. The compaction and mineral cementation during the early diagenetic phase are main controlling factors for the reduction of the porosity volumes of the reservoir sandstones, whereas dissolution of framework grains especially the dissolution of feldspar during the late diagenetic phase is interpreted as the key diagenetic controls for the formation of the excellent reservoirs.