The flow visualization of two vertical-axis-wind turbines, Savonius and Cyclogyro rotors, has been performed by using a three-dimensional smoked tunnel, a system of still cameras and a set of strobo-lighting systems. It can be seen by observing the flow pattern for nonrotating turbines that the driving torque at the initial stage of rotation is generated by the lift of one blade and the drag of other blade in the Savonious rotor for almost every azimuth angle whereas the driving torque is very small in the Cyclogyro rotor. For rotating turbines, it can be observed that the sinusoidal change of lift and drag of the rotors generates a series of shed vortices like Karman vortex streets and the flow separation and reattachment on the rotor blades occur cyclically through the rotation in both types of rotors.