Abstract: Although it was often proposed by a number of investigators that peak residual stresses be reduced when the sum of the residual and vibratory stresses exceeded the yield point of the material, there is noevidence observed from studies to prove that the vibration ageing would cause the attendant plastic deformation. Based on the dislocation theory, it was explained that after vibration ageing the relief degree of the residual stress depended on the change of the dislocation density of the metallic material. Because of the change of the dislocation density, the resistance to plastic flow was increased and the dimension stability of weldment was improved.