Oxygen transfer and the energy dissipation by the flow types at the stepped weir structure were performed through hydraulic experiments. Nappe flow occurs at low flow rates and for relatively small step slope. Dominant features of an air pocket, nappe impact and subsequent hydraulic jump occurs. At larger flow rates, skimming flow occurs with formation of recirculating vortices. Transition flow showed simultaneous occurrence of skimming flow at upper steps and nappe flow at lower steps. Air entrainment occurs through free-falling nappe impact and subsequent hydraulic jump in the nappe flow, and occurs from the step edges in the skimming flow. Energy dissipation occurs through the jet impact and the subsequent hydraulic jump in the nappe flow and occurs through maintaining the recirculation vortices between step edges in the skimming flow regimes. The average values of the oxygen transfer are 0.45 in the nappe flow and 0.28 in the skimming flow, and the efficiencies of energy dissipation in the nappe flow and in the skimming flow are about 70~95(%) and 60~90(%), respectively. From these results, the stepped weir structure is found to be efficient for oxygen transfer and for energy dissipation. KeywordsOxygen transfer; Energy dissipation; Nappe flow; Skimming flow; Stepped weir