Tidal stream is expected as a high-integrity energy source in the world because the time change of flow direction and flow velocity are predictable. Tidal stream 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 conversions 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 one 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. The capability to set the rotational planes of tandem propellers equal to the flow direction is required for high output power. In order to keep the posture stable in every circumstance, the winglets were installed on each rear blade. In this research, effectiveness of winglets in case mooring with a wire at near the water surface was verified by the circular water channel test. Besides, the performances in two cases with and without winglet were analyzed by 3-D steady numerical simulation.