The purpose of this study is to analyze the interface debonding of RC beams strengthened by carbon fiber sheet(CFS). The behavior of damaged RC beams strengthened with CFS is analytically investigated next using linear elastic fracture mechanics(LEFM) approach and the finite element method. The study includes an investigation of the separation mode by interface fracture of the strengthening materials due to the interfacial shear and normal stresses. The numerical method is presented to obtain the value of interfacial fracture parameter such as the strain energy release rate. Based on the results of this study, it is found that the critical case occurs when the interfacial cracks occur within a short region of the flexural crack. The CFS strengthening has not an adequate factor of safety against interfacial debonding of CFS. Furthermore, for the thicknesses of the adhesive studied[1mm~3mm], it is no noticeable effect on the strain energy release rate.