Carbon fiber reinforced silicon carbide matrix composites (C/SiC) are promising candidate materials for high-temperature structural applications. However, in oxidizing environments the two main constituents, that is, carbon fiber and pyrolytic carbon interphase which bears and transfers loads respectively are susceptible to deplete rapidly for oxidation. In this paper, the oxidation behavior of carbon fiber and pyrolytic carbon were investigated by simulating environmental experiments and scanning electron microscopy. The reactivity discrepancy in the carbonaceous constituents and in the different zone of carbon fiber was discerned. After oxidation, the morphology of carbon phase broken before oxidation were compared with that of those broken after oxidation. Based on the microstructural model, the contrast results of morphology were well interpreted from the reactive preference and selectivity.