A modified surface layer was fabricated on Ti-6Al-4V alloy by means of wet peening treatment. The fatigue life of specimen which suffered wet peening treatment was obviously increased. Compared with the fracture of un-peened sample, the fatigue crack initial site was transferred from surface to the interior region beneath the surface (about 1 mm depth). The co-action of grain refinement strengthening and dislocation strengthening dominated the transferred fatigue crack initial site. The compressive residual stress effectively delayed the crack propagation. The grain refinement strengthening, the dislocation strengthening as well as the compressive residual stress were the main factors of increased fatigue life.