The aim of this paper was to present preliminary results of researches conducted in PZL Mielec within the FAST_FSW project (INNOLOT program) on influence of tool eccentricity on microstructure and mechanical properties of 1.6 mm and 0.8 mm thick Al 7075 T6 sheets joints. Samples were performed using CNC milling machine. Range of tool eccentricity was 0-0.3 mm, rotational speed and feed rate were variable. The influence of applied welding parameters on microstructure and mechanical properties of joints was assessed on the basis of metallographic inspections and tensile tests. Microstructural observations have shown that the change of FSW tool eccentricity affects the weld geometry: the mixing zone depth, the weld width, or the shape of the contact line.