<p>In this article, the post-local-buckling behavior and ultimate strength of symmetric and unsymmetric cross-ply composite plate structures with a box cross section under uniform end shortening have been investigated using the finite strip method (FSM). The first-order shear deformation theory in conjunction with the minimum potential energy principle was employed. The boundary conditions at the loading edges are simply supported. Hashin and Rotem’s failure criterion has been implemented for failure analysis, and once the failure occurs, the properties are assumed to degrade instantaneously. Two different models have been considered to apply material properties degradation, i.e., the complete ply degradation model (CPDM) and the strip degradation model (SDM). In the first model, the properties of the entire ply are degraded instantaneously after the failure, and in the SDM, the degradation of material properties occurs only in the strip where the failure has taken place. Convergence analysis has been performed for the number of finite strips as well as the number of terms in the displacement field in the FSM. The finite element method (FEM) is used to verify the results. The effects of the flange-to-web width ratio as well as the thickness ratio on the ultimate strength and the post-local-buckling behavior of the structures have been investigated. It is noticed that the first ply failure (FPF) load as well as the last ply failure (LPF) load of structures with symmetrical layup are higher than the corresponding loads for the unsymmetrical layup.</p>