Experimental evidence demonstrates that diglycidyl ether of bisphenol-A (DGEBA)/meta phenylenediamine (m-PDA) epoxy resin matrix used in the single fiber fragmentation tests exhibits nonlinear stress strain behavior in the region where E-glass fiber fracture occurs. In addition, strain hardening after the onset of yield is observed. Therefore, linear elastic shear-lag models and the Kelly-Tyson model are inappropriate for the determination of the interfacial shear strength for this epoxy resin system. Using a strain-dependent secant modulus in the Cox model, the calculated interfacial shear strength is shown to be relatively lower by at least 15% than the value determined using a linear elastic modulus. This decrease is consistent with numerical simulations which show the linear elastic approximation over predicts the number of fragments in the fragmentation test. In addition, the value obtained by the strain-dependent secant modulus is approximately 300% relatively higher than the value predicted by the Kelly-Tyson model.