Abstract: The single-side welding process of friction stir welding(FSW) was used on 30 mm-thick plate of 5A01 aluminium alloy. Microstructures and mechanical properties of welded joint were investigated. The results show that appearance of the welded joint is in good shape; the welded joint can be divided into three regions by microstructures: weld nugget zone, thermo-mechanically affected zone and heat-affected zone; the weld nugget zone consists of top weld nugget and bottom weld nugget, which are differently affected by heat; the forming of the “onion ring” structure which appears at weld nugget zone is considered to be related to the three machining planes of stirring pin; the thermo-mechanically affected zone is affected by various degrees of thermal mechanical action and thermal cycling along the thickness direction, so its top, middle and bottom regions present different organizational characteristics; the mechanical properties of welded joint are good: the tensile strength of welded joint is more than 90% of that of the base metal and the tensile fracture occurs in the welded joint.