Carbon fiber reinforced SiCN matrix composite (C/SiCN) was used in present investigation, its amorphous SiCN matrix was derived from the hexamethyldisilazane by chemical-liquid and vapor-infiltration into the carbon fiber weaving preform. The tension-tension fatigue was conducted at 1300°C in vaccum both for the as-received and 1500°C-annealed C/SiCN with 60Hz and 41MPa. The microstructure evolution of SiCN during fatigue was examined by X-Ray diffraction and transmission electron microscopy (TEM). The results indicate that pronounced crystallization takes place in the as-received C/SiCN during fatigue, and only β-SiC crystallites are detected within amorphous SiCN matrix, Si3N4 can’t be detected. This is different from the literature that reports that there isn’t any crystallization when C/SiCN is annealed at 1300°C under nitrogen or argon atmosphere. As for 1500°C-annealed C/SiCN, there isn’t any crystallization during fatigue, and it can be attributed to the phase stabilization in the SiCN matrix after annealing. Both vacuum and fatigue stress promote the crystallization course because they accelerate the decomposition of amorphous SiCN and atomic diffusion. Degradation – crystallization mechanism is used to explain the crystallization behavior of amorphous SiCN under condition of vacuum and tension-tension fatigue.