A flapping water turbine is a kind of power generating machine which extracts mechanical energy from flowing water by swing of blade motion. In this study, we carried out numerical simulation of blade motion and experiment on river. In the simulation, we made numerical integration of momentum equation about roll angle and compared the result with experiment in water channel. The disagreement in roll angle between the simulation and the result of experiment were discussed by using analytical model and basic data of blade fluid dynamic characteristics. The result of river experiment showed that flapping water turbine can extract energy from stream with rippling surface and turbulence.