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The use of high grade steels shall permit to design and to manufacturelighter structures made by a large use of welded plates such ships, bridges, off shorestructures etc., increasing their safety too. A large experimental activity has beenscheduled carrying out fatigue tensile tests on a great number of welded joint full sizethickness specimens. Butt and transverse stiffener welded joints, both under constantand variable amplitude loads have been considered. Four steel plates, thickness 10 and30mm, have been considered in three strength levels: standard grade (S355N,normalized and S355M, with thermo-mechanical treatment), medium grade (S690Q)and a high strength steel (S960Q). In order to investigate the crack path and thesubsequent failure of the welded joint a metallographic analysis has been carried out.The assessment of the fatigue behaviour by Miner rules has been unsuccessful. Bymaking changes to Miner damage rule and taking into account of the Double LinearDamage Rule (DLDR) meaningful estimates of the fatigue life have been obtained.