In order to investigate the tension-tension fatigue behavior of C/SiC composite fasteners,the fatigue life under different stress levels was obtained when the stress ratio was 0.1 and the loading frequency was 10 Hz. Using the fracture analysis and microstructure observation method,the meso-mechanism of fatigue failure of C/SiC composite fasteners was also analyzed.The results indicate that there are two types of failure modes for the tension-tension fatigue behavior including thread breaking and thread pulling off.An empirical two-parameter power exponent form of formula characterizing the fatigue life is proposed,and fatigue life for the two failure modes are similar.The fatigue limit for C/SiC composite fasteners is 65% to 70% of tensile strength,which means that if the maximum fatigue stress is more than 70% σ_max,the damage of C/SiC composite material significantly increases with cyclic number.