The fatigue failure of TC17 alloy was investigated under asymmetric loading with the help of piezoelectric fatigue tester (20kHz) and electromagnetic resonance fatigue tester (110Hz)

and could be divided into surface induced failure mode and interior induced failure mode. Micro cracks near the center cross section of the specimen surface caused by slip trace under cyclic loading and the machine defects were the main reasons that caused the surface induced fatigue failure. The interior induced failure was mainly caused by the cleavage fracture of primary α phase. The frequency effect on the fatigue strength and fatigue failure mechanism was not obvious. And the variation of the failure mode resulted in a bilinear stree-fatigue life (S-N) curve of TC17 alloy. The formation of interior fracture could be divided into three stages: cleavage fracture of primary α phase

short crack propagation and long crack propagation. The critical stress intensity range of long crack propagation can be determined as 3.3MPa·m1/2 under the stress ratio of 0.1 at room temperature.