Stiffener/skin structures in integrated composite parts suffer debonding between stiffener and skin due to loading. Application of fracture mechanics to assess structural integrity of the integrated composite parts is difficult because of complexity of the structural configuration and inhomogeneity of composite materials. A simulation method to assess the structural integrity that predicts onset and growth of debonding and estimates the stiffness degradation due to debonding is needed. In this study, a simulation method based on FEM is investigated for stiffener/skin structures. The simula-tion method is intended to be versatile and easy to implement for commercial FEM codes. Stiffener/skin structures are modeled as cohesion of shell elements. If debonding is judged by an energy release rate criterion, the cohesion is released. Delamination growth in a DCB specimen and an ELS specimen is simulated and compared with theoretical and experimental results to demonstrate the feasibility. Thus, the debonding of stiffener/skin structures is simulated, and is reproduced success-fully by the proposed method.