Research and development of a floating-submersible type tidal current power generation system which can be used with low cost and in low speed tidal current is introduced. The system consists of the power generator by a horizontal axis turbine with the “Lens” diffuser and a slack mooring system with four ropes. The “Lens” diffuser has a large buoyancy so that the power generator floats on the free-surface when the tidal current is small but it submerges in the middle layer to generate power when the tidal current is greater than certain speed. The power generator changes its direction passively according to the direction of tidal current. Results of experiments by using 1/6 scale model to confirm the performance in the reciprocating flow in the towing tank are reported in the presentation.