Surface stress engineering processes aim to improve the life of components through the generation of favorable compressive residual stress fields by locally plastically deforming the near surface region. Peening is the most common means of surface stress engineering to prolong life. Ultrasonic peening treatment (UPT) use high-frequency acoustic force (20¿30 kHz) is applied, often by means of a portable tool, to the surface in order to introduce beneficial compressive stresses. This method currently used in offshore installations in the UK and worldwide as a method to extend the fatigue life of high stressed welds of structural integrity concern. The objectives of this proposed study is to investigate the depth profile of beneficial compressive residual stress introduced by successful UPT in stainless steel, as well as medium and hard strength carbon steel.