The postbuckling behavior and progressive failure of thin-walled box-shape composite beams (BSCBs) have been studied using a simplified composite panel model. It is shown that the carrying capacity of BSCBs under bending and torsion mainly depends on the postbuckling and progressive failure of panels. It is often necessary to identify and follow secondary buckling of tailored panels for correct estimation of carrying capacity. The failure process of stiffened composite panels is simulated in following path. The criterion of local failure in flange of stiffeners is applied to predict the ultimate failure of stiffened composite panels and thin-walled BSCBs. The comparison of the modeling and test shows that the prediction of BSCBs failure is satisfactory.