Abstract: There are braid river and meandering river sediments of Chang 2 and Chang 3 oil reservoirs in the Ansai oilfield. The channel sandstone bodies are in NE-SW striking zoning distribution, controlled by sedimentary microfacies, and the sand bodies of river channel are the main reservoirs for the hydrocarbon. Syngenetic open space and vugular-solution pores are main pore types of Chang 2 and Chang 3 oil reservoir sandstones in the Ansai oilfield. Chang 2 and Chang 3 sandstone belongs to the medium or fairly good reservoir whose pore-throat structure belongs mainly to medium-thin throat-eyelet type and thin-small throat- eyelet type. The reservoir quality of the Upper Triassic Chang 2 and Chang 3 sandstones of the Ansai oilfield in Ordos Basin was strongly influenced by the diagenetic events which mainly included mechanical-chemical impaction, consolidation and dissolution．The mechanical-chemical impaction and consolidation were the main factors causing the loss of pores in the sandstones．In the cements，quartz-feldspar overgrowths and carbonate were the main cements that reduced the reservoir quality of the sandstones．Thin chlorite film around detrital grains partially prevented quartz and feldspar overgrowth and carbonate cement precipitation，which caused the preservation of a part of primary pores and hence resulted in a better reservoir quality and the decrease of intergranular space. The main factors for the formation of great quantities of secondary pores were the dissolution of detrital feldspars and part of cements by the meteoric water during epidiagenesis and the acidic liquid related to decomposition of organic mater during hydrocarbon enrichment at the late diagenetic stage. There are many barriers and interbeds in Chang 2 and Chang 3 oil reservoirs, and the sedimentary rhythms are composed of positive rhythms and reverse rhythms which result in the heterogeneity of the competent reservoir, in which the heterogeneity of Chang 3 oil reservoir is stronger than that of Chang 2 oil reservoir in the Ansai oilfield.