The object of this research is to clarify the stiffening effect of relatively large-sized hat-shaped stiffeners on a plate which are often utilized in FRP ship constructions. In the previous reports, both the bending and buckling deformation of a fundamental single stiffener model was described and it was shown that the lower width of a hat-shaped stiffener is the most important parameter in view of the stiffening effect on a plate. It was also shown that a simple model considering the rotational spring constant of a stiffener web gives a good explanation of the local buckling mode as well as the bending deformation of a fundamental symmetric structural model. One of the main structural characteristics of a hat-shaped stiffener is that the effect of its torsional rigidity cannot be neglected since it is attached to a plate along two separated lines and makes a closed section. In this report, an analytical model considering two joint lines of stiffener webs to a plate is proposed to modelize the stiffening effect of the torsional rigidity of a hat-shaped stiffener on the bending deformation of a plate. In the first part of this report, the bending deformation of a unidirectionally stiffened plate is analyzed by means of Ritz method on a simply supported orthotropic plate with a single or double hat-shaped stiffeners under uniform lateral load and compared with detailed finite strip analyses. It is shown that the stiffening effect depends greatly on the position on a plate where a stiffener is attached and that the effective torsional rigidity is reduced by the cross-sectional deformation of a stiffener. In the second part of this report, the bending deformation of a stiffened plate with multiple hat-shaped stiffeners is analyzed and compared with an equivalent orthotropic plate model using an effective torsional stiffness. It is made clear that the deformation behavior cannot be well explained by the conventional equivalent rigidity method on a stiffened plate with multiple hat-shaped stiffeners.