Recently, several power plants from the rotation of turbine with tidal current have been tried. Since the density of seawater is 800 times as much as that of the air, the loading of water on a turbine strictly requires much more strength and stiffness of blade compared with the wind turbine. Neither wind turbine nor standard hydroelectric dam turbines can simply be submerged into an ocean current. There are some formidable technical challenges to be overcome compared with the wind turbine. Key issues are cost effectiveness, structural integrity and workability in access and installation. The metal blade has enough strength, but is too heavy to install and handle easily. The light weight and extreme strength are essential to the blade. The objective of this work is to determine the mechanical properties of the tidal turbine, and to examine the availability of the turbine blade of composite materials for an approach to eliminate the above problems. The study was conducted in the preliminary study of the demonstration plant, which will be settled in Oma Promontory, Aomori Prefecture in Japan, whose maximum power output is 300kW and turbine diameter is 11 meters. A number of materials were considered, i.e. comprised rolled steel, aluminum bronze, GFRP for blade. We made two models for structural study based on the propeller blade shape with thin section and the wind turbine blade shape with thick section. The FEM analysis were conducted as follows, Aluminum-Bronze solid model with propeller shape; the real model at the present moment in the Oma plant. Composite material solid model; same shape as propeller but applied with composite materials. Composite material shell model with wind turbine blade Shape; structured by monocoque construction with changing the thickness by 10mm from 10mm to 50mm. The properties of GFRP for the structural study were measured from the ISO-laminates, which were fabricated by VaRTM, of multi-axial non-crimp fabrics and epoxy. Furthermore, the vibratory cavitation erosion tests of Composite materials were conducted. In order to compare with the aluminum bronze and composite, each cavitations weight loss in fresh-water were measured and observed. As the result, the multi-axial GFRP for propeller type blade was insufficient in rigidity and strength of shear. It is necessary to use not GFRP but CFRP for the propeller type blade. In contrast, as for wind turbine type blade, it was led to the conclusion GFRP blade is workable. As for erosion, the durability of composite materials is remarkably inferior to metals.