Abstract: The finite strip method is extended to include the nonlinearities for the material and geometry of initially deflected box sections under a constant compression eccentricity and a constant load eccentricity, respectively. The effect of enhancing of yield stress in corners of the cross—section due to coldworking and the behariour of smooth transition of stress—strain curves from elastic to plastic range on collapse load of box sections is taken into account in post—buckling range. A test programme of compressive box sections was carried out and the comparison of experimental with theoretical results shows a good agreement.