Abstract: The post-buckling and carrying capacity of thin-walled box-like structures under bending and twisting was predicted by the finite element analysis of stiffened composite panels. Based on a revised first-order shear theory, the panels were considered as Mindlin plates supported simply along four edges, which remained straight during deformation. The arc-length method and branch-switching technique with asymptotic analysis were applied to postbuckling path tracking. Three states of progressive failure were defined and the postbuckling and carrying capacity of stiffened panels were determined considering the influence of lamina failure on stiffness in accordance to Hoffman criterion. The numerical results showed that carrying capacity of the stiffened composite panels is considerably high in postbuckling states, and a good agreement was found between the prediction of carrying capacity and the experimental results of the composite box-like structures under bending and twisting.