The stress field in a postbuckled I-section composite stiffener under compression is presented. The flange and web elements are symmetric orthotropic laminates and the loaded ends are simply supported. The analysis is based upon a higher-order shear deformation theory. By assuming trigonometric functions in the loading direction for the displacement field, the nonlinear large deflection equations are converted into simultaneous first-order nonlinear ordinary differential equations. These equations can be solved numerically for any boundary condition along the unloaded edges using a two-point boundary value problem solver. Results show that peel stress initiates at the free-edge of the flange. The locations of the maximum peel stress coincide with the failure sites observed in previous experiments.