The interface properties between carbon fiber with different degrees of surface oxidation and isotactic polypropylene matrix have been investigated by using the fragmentation test on single-fiber composites. The exothermic melting peak of spherulite and transcrystallinity has suggested that the transcrystallinity has grown earlier than the spherulite. It has been found that oxidation of carbon fiber reduces the fragment length. The length is also influenced by the interface morphology. While fragment length increases with increment of fragmentation test temperature, the effect of the surface treatment can still be evaluated by the fragmentation test.