The majority of fatigue tests of welded specimens is based on shop-made samples generally exhibiting minor misalignment. Due to the challenge of ensuring misalignment-free joints in industrial manufacturing processes, investigations focussing on the effect of misalignment on the fatigue strength are important. Therefore, this paper deals with the influence of axial misalignment on the fatigue resistance of butt-welded ultra high-strength steel specimen. In addition, the effect of high frequency mechanical impact treatment (HFMI) on the fatigue performance is researched. In the course of the experimental investigations, specimens exhibiting three different levels of axial misalignment are manufactured. Fatigue tests at a stress ratio of R = 0.1 in as-welded condition reveal a significant drop in fatigue strength with increasing axial misalignment. Fatigue assessments of the as-welded test results based on nominal, structural and effective notch stress approach are performed taking into account the sample-dependent misalignment factor. The given equations enable an improved consideration of axial misalignment regarding to fatigue strength. The HFMI treatment increases the fatigue strength compared with the as-welded state; the detrimental effect of misalignment is less pronounced. A comparison to the current IIW guideline for HFMI treatment reveals a conservative assessment if IIW-recommended FAT values for as-welded condition are applied.