C/SiC composites had been prepared by PIP process. The evolution of microstructure and mechanical properties after high-temperature (>1500 °C) oxidation were discussed and compared. The results showed that PIP-C/SiC composites had been severely damaged after oxidation tests, micro-cracks and holes emerged along with the increasing of the oxidation temperature, which lead to the decreasing of mechanical properties. After high-temperature flexural tests, the thermal mismatch between C fibers and SiC matrix gradually caused the expansion of micro-cracks, which lead to the decreasing of flexural strength. The mechanical performance deteriorated because defects in the matrix increased and some fibers were oxidized. For long-time use at higher temperature (≥1600 °C), it was necessary to develop a more protective coating with higher stability and melting temperature.