The fracture toughness increases but the long-term oxidation resistance of carbon short fiber reinforced C/SiC composites decreases if the volume fraction of carbon fibers is high. To understand the oxidation kinetic and their dependence from the C/SiC microstructure the composites and also the used carbon short fiber bundles were investigated by a thermo balance and by scanning electron microscopy. Additionally the oxidation of single carbon short fibers was measured in situ by a hot stage light microscope. To simulate oxidation kinetics a software program using Finite Element methods was developed. Below 800 °C oxidation resistance can be improved by increasing the degree of graphitization of the carbon material within the short fiber bundles. Above 800 °C the number of junctions between the short fiber bundles has to be reduced.