The bi-directional impulse turbine and the bi-directional diffuser for tidal energy conversion is investigated in this paper. The bi-directional impulse turbine with fixed guide vanes is adopted because the turbine has a large efficiency and an advantage of maintenance. The turbine characteristics is obtained experimentally by the air turbine test rig. As a bi-directional diffuser suitable for the bi-directional impulse turbine, the diffuser geometry is proposed, which has a simple cone geometry. The combination system of the impulse turbine and the diffuser is investigated experimentally by using the water tunnel at IOES. This system is proved to produce the power by a tidal flow experimentally.