The article presents the results of tests focused on the effect of stress relief annealing on the mechanical properties and fatigue strength of joints made of S420MC steel grade belonging to a group of thermo-mechanically control processed steels. The article contains the description of the aforesaid tests and presents the results of the basic mechanical tests as well as of internal stress measurements. The text also presents information about fatigue categories experimentally determined for the four most popular types of welded joints at their initial state and after stress relief annealing. In addition, the article informs that the stress relief annealing process recommended by German guidelines SEW 088 does not result in an increase of the fatigue strength of S420MC steel welded joints.