Abstract:The ultrasonic shot peening (USSP) was used to treat TC4 surface, the samples were loaded by four-point-bending fatigue test, and residual stress distribution before and after fatigue loading was tested by X-ray stress tester. By TEM equipment, microstructure of TC4 before and after fatigue loading was characterized. The results showed that the maximum residual compressive stress of surface nanocrystallization TC4 exceeded its yield strength; When the loads were higher than fatigue limit, the residual stress released and the residual stress field decreased significantly; When the loads were close to or lower than the fatigue limit, the residual stress field tended to be stable and was no longer evidently altered with the change of cyclic time and loads; The configuration and quantity of dislocations and dislocation cells were changed in the process of fatigue, the decrease of the dislocation density resulted in the relaxation of residual compressive stress.