The object of this research is to clarify the stiffening effect of a hat-shaped stiffener on a FRP single-skin panel. In the previous report, the bending deformation of a single stiffener model was described. However the ultimate tensile and compressive strength of FRP laminates are approximately equal to the yield strength of mild steel, the Young's modulus of them is generally only one tenth to one twentieth of that of steel, so that a problem of elastic instability is not less serious than that of bending. In this report, the buckling deformation is analyzed on a simply supported orthotropic plate with a single hat-shaped longitudinal stiffener under uniaxial compression. An instability may occur in the form of local or overall buckling in such a fundamental plate structure considered, so that three analytical models are proposed to modelize the stiffening effect of a hat-shaped stiffener, that is, Euler buckling, local wrinkling and the rotational spring model proposed in the previous report. They are compared with detailed finite strip analyses and experimental results on plastic models. It is 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 as shown in the previous report. The difference between buckling deformation mechanisms of hat-shaped and T-shaped stiffeners is also demonstrated based on these results.