Application of high frequency mechanical impact (HFMI) treatment for fatigue strength improvement of welded structures becomes more important and is already widespread in industrial applications. Thereby a hardened steel pin deforms the weld toe plastically and causes compressive residual stresses, a reduced notch effect, a removal of surface imperfections and a local hardening of the material. At present, the benefit of this post-treatment technique is only included in global approaches, an evaluation for complex structures using local concepts is not yet considered. In this work, at first, experimental investigations of one specific HFMI-process provides information about the acting dynamic forces and indentation depths. The determined data acts as input parameters for a numerical simulation loop, including a structural weld simulation, a numerical investigation of the HFMI-treatment and an estimation of the local fatigue behaviour. Structural weld simulation based on experimental data is made to analyse the residual stress condition and heterogeneous microstructure after the joining process. Based on these results a numerical computation of the HFMI-process is set-up. An investigation of significant process parameters shows the influence of the post-treatment on the local weld toe topography and induced residual stress state. X-Ray measurements of residual stresses at welded and HFMI-treated specimens are analysed to verify the numerical results, whereby a good accordance is achieved. Finally, a numerical estimation of the local fatigue life is performed to compare the critical regions of failure in the as-welded to the HFMI-treated condition. An analysis of the reduced notch effect and the compressive residual stresses on local fatigue completes the work.