Hat-shaped stiffeners have often been adopted in FRP ship constructions, not only because they are easily moulded on FRP hull laminates, but because their structural form is effective to stiffen the low rigidity of a FRP single-skin panel. However this stiffening method has not been applied to steel ships, so that there are few design data available for this type of stiffeners. The difference between hat-shaped and T-shaped stiffeners is that the former is attached to a plate along two separate lines making a closed section which induced a considerable tortional rigidity. The object of this research is to clarify the deformation mechanism of a FRP single-skin panel stiffened with hat-shaped stiffeners in view of obtaining fundamental design data. In this report, the bending deformation of a fundamental structure is analyzed on a simply supported orthotropic plate stiffened with a single hat-shaped stiffener under uniform lateral load. Three analytical models are proposed to modelize the stiffening effect of a stiffener. They are compared with detailed finite strip analyses on a fundamental structure and experimental results on plastic models. It is shown that a simple model considering the effective breadth of a plate and the rotational spring constant of a stiffener web gives a good explanation of the bending deformation of a fundamental structure.