Tidal current is expected as an attractive renewable energy source because the time change of flow direction and velocity is predictable. Tidal current energy extraction devices can be categorized as a horizontal axis system and a vertical axis system. The authors have developed a horizontal axis counter-rotating type power unit which is applied in the conversion of wind energy and tidal energy. A counter-rotating type tidal stream power unit is composed of tandem propellers driving double rotational armatures, while the rotating torques are counter-balanced in the power unit. The power unit can be moored with a cable and keep the posture stable without rolling motion in the tidal stream, in which the cable is hooked at the balancing point of the moment among the buoyancy, gravity and drag. The posture, however, may lose balance by a dynamic disturbance. In order to keep the posture horizontal in every circumstance, the winglets are installed on each rear blade. In this research, effectiveness of winglets are verified by a circular water channel test. Besides, the numerical simulation was carried out to investigate the flow behavior around the propellers.