A postbuckling analysis for composite stiffened structures is a representative nonlinear analysis, but is difficult to obtain analysis results and a lengthy computational time is required. In this study, an accurate and realistic postbuckling model for a stiffened composite panel is made, and optimization module is studied. In order to predict the realistic behavior in the postbuckling region, cohesive elements were introduced to the analysis model to account for skin-stiffener debonding. The global behaviors and failure loads of the analysis results were found to be similar to the experimental results. Based on this analysis model, the optimal layer design of a composite stiffened panel up to postbuckling failure was performed using micro genetic algorithm. The optimized result showed a higher maximum failure load in the postbuckling region.