Investigations on welded joints of a low strength steel and of an age-hardened aluminium-alloy have been performed to compare the effects of different post-weld treatment methods. The experimental results show, that the methods which are working with help of ultrasonic activation lead to a higher depth of penetration of the compressive residual stresses as shot peening does. The cold worked surface area is deeper and the hardening effect can be found in deeper layers. Furthermore the shape of the weld toe is changed strongly due to the cold working of the surface with the consequence of a change of the weld toe geometry. Anyway the fatigue strength improvement is comparable to that which is generated by shot peening (steel) or less than after shot peening (aluminium). The reason is, that the shape of the treated weld toe profile must not be necessarily smoother due to the ultrasonic hammer peening. Depending on the treatment parameters also sharp defects can be produced at the boarder of the treated zone which may compensate the beneficial effect of the compressive residual stresses.