Abstract: Microstructure and mechanical properties of 5083 aluminum alloy friction stir welding (FSW) joints were researched. The microstructure of each welding region was observed by optical microscopy, and the micro-hardness and mechanical properties were determined. The results show that the microstructure of weld nugget is tiny dynamic recrystallizatal grain, and strengthen phase distributed evenly. Because of the effect of dynamic recrystallization and the welding heat input, the mechanical heataffected zone changed greatly, and the grain grows a lot, while the strengthen phase refined. The heat-affected zone influenced by welding heat input only, so grains coarsen and the strengthen phases aggregate. The hardness of friction stir welded joint is almost equal to that of the base metal. Owing to the welding heat input, the hardness of TMAZ and HAZ decrease. The tensile and impact properties of FSW joints are not less than that of the base metal, which tensile specimens were broken in the base metal, and the impact of joints reached more than 1. 5 times of the base metal. The impact value of heat affected zone match to base metal considerably.