The demand of stronger and lighter structures is rapidly growing. Today's applications vary over a large range of industries where the requirements are different for different products which requires application driven research of the 3rdgeneration Advance High-Strength Steels (AHSS). These are high-strength steels that are tailor-made to get desired mechanical properties. One AHSS manufactured by SSAB is their Strenx® 1100Plus steel that is specially designed to get excellent welding properties. In the current study the fatigue life improvement was investigated when using post weld treatments, TIG dressing and HFMI (High Frequency Mechanical Impact), for butt welded joints in Strenx® 1100Plus. Fatigue testing and assessment was carried out, using nominal stress and effective notch stress approaches, and the results were compared with fatigue strength recommendations for similar joints and steel grades given by the International institute of Welding (IIW). It is concluded that one additional FAT class increase could be observed in comparison to what is recommended by IIW for both types of post weld treatment methods.