Abstract: The macro-and micro-structures and micro-hardness of friction stir welding of 7A52 aluminum alloy were investigated.The welded joint of aluminum alloy 7A52 can be divided into three regions as follows:heat affected zone,(HAZ),thermo-mechanically affected zone,(TMAZ) and weld nugget.The weld nugget had a dynamic recrystallized,fine isometric crystal structure.The TMAZ had a highly deformed structure,and the grains were curved strip and larger than that in the weld nugget.The HAZ had similar structure to the base metal,but the size of grains was larger than that in the base metal.Distribution of microhardness in the cross section presented a trend of high-low-high-low-high.The base metal outside the weld show the maximal hardness,and the top of the weld nugget has the same hardness as the base metal,the lowest hardness was in the advancing side in the HAZ.