Recently the aircraft and military industries widely have been using aluminum alloys particularly because of their fine strength to weight ratio. However in compare with steels they represent welding difficulties and also lower ductility. In last years it has been observed that Friction Stir Welding (FSW) method represents better microstructure and mechanical properties than conventional methods in welding aluminum alloys. In this study experiments were performed to investigate the effects of FSW process parameters including rotational and welding speed on the microstructure and mechanical properties of aluminum 5083 alloy in lap joint welding and different joint defects were analyzed. It was observed that the nugget area had the best grain size and also higher hardness in compare with the other welding areas. Also the best joint properties were achieved at the rotational speed of 825rpm and welding speed of 32mm/min.