In order to clarity the aerodynamic properties and performance of Magnus wind turbines with spiral fins, numerical simulation were conducted. The power coefficient of the wind turbine depends on the velocity of the spanwaise flow around cylinders which induced spiral fins on the surface of the cylinders. The maximum power coefficient of the turbine can be obtained at the spanwise flow velocities are almost the half of the uniform velocity. In the case of the field experiments of the Magnus wind turbines, spiral fins generate the internal flow around cylinders. The numerical simulation has same tendency of the field experiments. The result seems to be the axial flow velocities play an important role in estimation and improvement of the performance of the Magnus wind turbines.