Surface hardening technology can make the parts form a surface gradient structure and generate high compressive residual stress, thus effectively improving the service life of parts. In recent years, the influence mechanism of gradient structure and residual stress on the fatigue life of parts has attracted great concern. The new development of the effects of gradient structure, residual stress and relaxation on the fatigue properties of parts was summarized. After the surface hardening, the grains on the material surface were fined obviously and distributed in a gradient form along the depth direction, thus promoting the crack sources to shift to the hardening layer. The superposition of residual stress and external force effectively reduced the actual stress of parts, thus affecting the fatigue life of parts. However, it cannot be revealed from the mechanism level. The main problem of residual stress relaxation is that the establishment of relaxation model is imperfect. In addition, during crack initiating, both gradient structure and residual stress have important influence on the fatigue strength of parts. However, which factor plays a leading role in this stage has not been assigned.