Composite members made of steel plate and concrete have been applied to marine structures in Japan. Torsion properties of the composite plates (open sandwich plate and sandwich plate) and reinforced concrete plate are investigated by static loading tests. Shear failure of concrete and yield of reinforced bar are observed in the loading tests. Cracks of the concrete is scattered but crack widths are larger than those of reinforced concrete plates. Hsu theory (truss theory considering concrete softening) is modified by authers and applied to the composite plates and compared to the loading test results. The modified Hsutheory showed good agreement with the test results as to deformation, strains and crack widths.