The present analysis deals with ultimate behaviors of a panel reinforced by a longitudinal rectangular stiffener under uniaxial compression. Their characteristics depend largely upon the flexural rigidity γ of the stiffener. There are two specific values γ₁, and γ₂ to classify the ultimate behaviors. In the case of a weak stiffener (γ<γ₁), the behaviors of the stiffened panel is similar to those of an unstiffened panel. In the case of a strong stiffener (γ₂<γ), the joint line is supported effectively by the stiffener, and local deformations dominate even at the ultimate strength. If the rigidity of the stiffener is intermediate (γ₁<γ<γ₂), local buckling occurrs first, but later on overall deformations become large. The panel reinforced by a one-sided stiffener shows rather complicated behaviors, since the effective neutral axis is influenced by local buckling and yielding in the stiffener.