In many cases, composite construction is adopted in FRP ship's hull incorporating plastic foams or woods, in order to compensate the low rigidity of the material. In such cases, the structural behaviour of the hull is very complicated for its structural anisotropy and heterogenity. The other factors to prevent the exact structural analysis of the hull are in the fact that the effectiveness of various members is rather ambiguous for the effects of the shear and the local deformation are large compared to the steel hull. For this reason, seven kinds of the simplified FRP composite structural models are fabricated and tested. The bending behaviour of these models is discussed from the view-point of the structural effectiveness. Simplified analyses are made on the effects of the shear deflection, the incomplete interaction of the stiffeners and the main beam, and the fall of effectiveness of the compression-side flange due to local deflection. Finally, the test-results of these models are compared and discussed, defining the structural efficiency as the ratio of the so-called structural Young's modulus and the material's one.