Abstract: The compositions,sedimentary structures,grain size distribution and sedimentary facies types are examined on the basis of cores,well logs and grain size data of the rocks from the 1st member of the Middle Jurassic Yan’an Formation in western Mahuangshan,Ordos Basin.The 1st member of the Yan’an Formation is built up of the fluvial sedimentary system in which the braided and meandering stream deposits are developed.Two sedimentary subfacies have been distinguished including the riverbed and overbank subfacies which may be subdivided into the riverbed lag,channel bar,point bar,overbank,backswamp and crevasse-splay microfacies.During the deposition of the Yan-10 oil measures,the 1st member of the Yan’an Formation was occupied by the braided stream deposits with well-developed channel bars and wider channels.The palaeocurrents were oriented NW to SE.Influenced by the lake-level rising during the deposition of the Yan-9 oil measures,the study area was prevailed over by the meandering stream deposits with the point bar deposits.