This paper presents a study on the ultimate capacity (maximum strength and plastic deformation capacity) of a thin–walled H–section beam. An experimental study is adopted by subjecting beams under cyclic loading to a large range of deformations with large depth–thickness ratios, which are over or close to the limit value of the current standards. The objectives of the present paper are to reveal the post local buckling behavior and to propose a handy method to evaluate the ultimate capacity. Instead of the commonly used parameter of width and depth–thickness ratio, the normalized thickness ratio WF developed in this study based on the author's previous studies can be regarded as the major parameter to evaluate the ultimate capacity. The relationship between WF and ultimate capacity is examined. The improved criteria for assessing the maximum strength and plastic deformation capacity of an H–section beam is proposed using normalized thickness ratio WF and is shown to be in good agreement with the experiment results.