The fiber/matrix interfacial strength in discontinuous carbon fiber reinforced thermoplastics was evaluated by a push-out test using a nanoindenter. Two types of composites were prepared. They were molded using polypropylene reinforced by discontinuous carbon fibers using different surface treatments. Thin polished specimens with a thickness of 20 μm were prepared. The load–displacement curves were obtained by extruding carbon fiber with a custom-made flat-ended push rod attached to the nanoindenter. The process of interfacial debonding fracture was numerically simulated by the cohesive zone model, using the mixed-mode critical energy release rate as a criterion. In addition, interfacial shear strengths were evaluated using the feature points of the measured load–displacement curve. It can be concluded from the results that the higher the interfacial shear strength, the higher the macroscopic tensile strength. Subsequently, Kelly–Tyson's theory was used to predict the macroscopic tensile strengths. The predicted tensile strengths of the composites were almost consistent with the practical values.
