Abstract: The damage evolution in 3D SiC_f/SiC composites in the fatigue-oxidation environment was investigated，and the failure mechanism of SiC_f/SiC in such environments was also discussed. The results indicate that the main damages in SiC_f/SiC composites in fatigue-oxidation environments include：cracking of SiC matrix；debonding，oxidation and ordering of pyrolytic carbon (PyC) interphase；fracture，oxidation and change in microstructure of the fiber. The composite interior is oxidized by the oxidizing atmosphere infiltrating the composites through the cracking matrix. The fibers are pulled out more easily due to the debonding and ordering of PyC interphase. The strength of the fibers is decreased because of the oxidation of the fibers，the increase of the amorphous carbon and the growth of the SiC grain in the SiC fiber.