The interfacial fracture observed in fragmentation tests performed with a model of polymer-matrix composite is predicted using an elasto-plastic shear-lag analysis and shear strength criterion. Interfacial debonding propagation is significantly affected by plastic deformation in the vicinity of the interfacial debonding tip. Hence, the matrix is assumed to be an elasto-plastic material that deforms according to J2 flow theory. The theoretical prediction of the interfacial debonding process is compared with the experimental results, which leads the value of the interfacial shear strength. Our recent energy-balance scheme, including an energy dissipation induced by the plastic deformation around the debonding tip, is used to calculate an energy-release rate to propagate the interfacial debonding. To validate the theoretical values of interfacial properties, FEM analysis employing cohesive elements is conducted, in which the theoretically obtained interfacial properties are introduced as strength and fracture toughness of the cohesive elements. Numerical prediction of the interfacial debonding propagation was proved to be in good agreement with the experimental, which illustrates the validity of the theoretically obtained interfacial properties.