Abstract: Effects of secondary annealing on fatigue performances of two groups of TC4ELI titanium alloys (annealed and secondary annealed) were studied.The results showed that secondary annealing could prolong the fatigue life of TC4ELI titanium alloy when the maximum fatigue stress was 0.8R_p0.2 or above, while the fatigue life changed little when the maximum fatigue stress was less than 0.8R_p0.2; that when the maximum fatigue stress was no lower than 0.8R_p0.2, the dislocation was prone to slip, and after the secondary annealing, the equiaxed α-structure of TC4ELI titanium alloy increasesd, β-transformed structure and the interface between the α/β phases decreased, and the dislocation slip path increasesd, which delayed the initiating of cracks; that when the maximum fatigue stress was lower than 0.8R_p0.2, secondary annealing reduced the lattice strength which resisted the movement of dislocation, and under the effects of lattice strength and dislocatial slip path, the fatigue life of crack at initiation stage didn't change significantly.