Based on analyses of base level changes and their effect on fluvial equilibrium profiles and sedimentation，as well as on field observations of the lower Permian Shanxi Formation in the Or— dos Basin，fluvial sequences and their relation to river types are discussed．The lowstand system tract is dominated by coarse—grained sandy to gravelly bed-load sediments which are deposited in braided river systems．They form amalgamated multistory，multilateral，and sheet—like sand bodies．The trasgressive system tract consists mainly of fine—grained suspension—load sediments， with isolated and lateraly ristricted sand bodies．It seems to be deposited in anastomosing river systems．When the rapid base level rise greatly exceeds the rate of the fluvial aggradation，lacus— trine or marine trasgressions may occur．The highstand system tract is dominated by a mixture of bed—load and suspension—load deposits generated mainly in meandering river systems，and in some cases，also in anastomosing rivers．Tectonic uplifts in the source area are likely to favor the for— marion of type一2 sequences．