Ultrasonic peening (UP) can improve the fatigue resistance of welded joints by introducing compressive residual stresses at the toe of the welds, and offers significant advantages over grinding in terms of speed and ease of application. The ultrasonic peening (UP) method directly deforms the surface at the toe of the welds with the use of ultrasonic vibrations and produces a 0.3 to 0.6 mm deep, by 3 to 5 mm wide groove with a uniformly smooth surface finish that is free from all traces of the original weld toe. UP is becoming a popular choice for fatigue enhancement in the shipbuilding industry. The tools needed to perform UP treatment are portable, easy to use, and user and environment friendly (little noise is produced). In this experiment we will characterize the residual stresses in ultrasonically-peened fillet welds.