Numerical analysis with the finite element method (FEM) was used in order to identify interfacial material constants of plain glass woven fabric composites under mode I loading. Relations between strength, stress intensity factor, and modulus in the interphase which were correlated with the onset of unstable fracture were determined. As a result, it was reasonable that the interfacial moduli were between 2.8 and 8.4 Cpa which corresponds to one to three times the resin modulus. It was suggested that the increase of concentration of silane coupling agent has led to an increase of the interfacial strength. Changing of fracture mechanisms from unstable to stable crack propagations was explained through the interfacial material constants.