Abstract: Ultrasonic Impact Treatment(UIT) has recently attracted considerable attention in the ship industry as an effective post-welding improvement method for welded joints. In order to evaluate the effect of UIT on the fatigue properties of welded joints, we propose a novel numerical analysis approach including welding simulation, dynamic elastic-plastic Finite Element Analysis(FEA) of the UIT process, and the evaluation of the fatigue strength of welded joints based on fracture mechanics. In the FEA model, the actual process parameters and ultrasonic-induced material softening, appropriately adjusted to fit the experimental results, are considered. The predicted residual stress distributions and fatigue strength of non-load-carrying cruciform joints before and after UIT are compared with the experimental results, and show a fairly high level of agreement.