Abstract Residual stresses exist in all manufacturing processes which use heat and/or force such as casting, forming, machining and welding. Sometimes they cause decrease in strength and life of components especially under dynamic loads and vibration conditions. To improve fatigue strength, a number of post treatment operations are being used such as grinding, shot peening, re-melting and heat treatment. Ultrasonic peening is a newly developed method for the improvement of fatigue strength of, mainly, welded joints and structures. By employing this process, geometry of weld toe can be modified for reducing the stress concentration. In addition, elimination of tensile residual stresses, exertion of compressive residual stresses and closing of cracks, voids and cavities are expected, too. The extra advantage of this technology is its application on massive and large structures which cannot be treated by other procedures. For investigating the effect of ultrasonic peening on stainless steel-304 welded parts, a series of experiments were designed and implemented. Ultrasonic peening is mostly used as a mechanical surface treatment method in the automotive and aerospace industries. However, this paper comprises the results of experimental fatigue strength tests along with metallography, micro hardness and corrosion resistance tests of welded pieces with and without processing by ultrasonic peening. Experiments proved that under post treatment by ultrasonic peening, a better mechanical and corrosion resistance is achieved.