High frequency mechanical impact (HFMI) is a post-weld treatment method for improving fatigue strength of welded joints. Much of the experimental research on fatigue performance of HFMI treated welded joints has concentrated on the beneficial compressive residual stresses created by the treatment and the fatigue strength of these joints under constant amplitude loading and relatively low mean stresses. Critical experimental data has been developed in a nearly completed European RFCS project. New mean stress and variable amplitude fatigue data are presented and evaluated with respect to a proposed International Institute of Welding (IIW) design guideline concerning HFMI treated welded joints.Peer reviewe