The compression, bending and tensile properties of C/SiC composites at elevated temperature air were tested. The microstructures of fractures at room temperature and elevated temperature were analyzed by scanning electron microscope, respectively. Results show that, when the test temperature varies from room temperature to 1 000 ℃, the compressive strength, flexural strength and tensile strength of C/SiC composites decreases from 247 MPa to 78 MPa, 480 MPa to 277 MPa and 247 MPa to 152 MPa, respectively. The performance of these three mechanical parameters of C/SiC composites is reduced by 68%, 42% and 38%, respectively. High temperature oxidation leads to interface degradation and carbon fiber structural damage, which aggravates the degree of fiber breakage, and changes the state of fiber-matrix bonding. As a result, the fiber toughening mechanism gradually disappears, and composite properties decays.