An analytical method to predict load-deformation curves of box-section steel beams and beam-columns governed by local-buckling is proposed. The box-section includes three types of manufacturing, i. e., corner welding, roll-forming, and press-forming. The predicted load-deformation relations can cover the post-buckling and deteriorating ranges. In this analyis, stress-strain relations of stub-columns are applied to the compressive part of the section under compression and bending. The stub-column behaviors employed in this analysis are statistically investigated on the basis of 112 specimens in the previous research. This analytical method is verified by comparing the analytical solutions with experimental results.