Abstract: Friction stir welding (FSW)and gas tungsten arc welding (GTAW)were used to weld AZ91D magnesium alloy sheets with the thickness of 2.2 mm.The influence of the welding parameters on the FSW weld quality and microstructure and mechanical properties was investigated.Sound weldment was obtained at the tool rotating speed of 1380 r/min, while the rotating speed of 1960 r/min was too high.The microstructure of each zone showed very different features,which depends on the thermal and mechanical conditions.The hardness and strength of the stir zone were remarkably improved due to the fine recrystallized grain structure.The mechanical properties of FSW weldments which were superior to those of GTAW weldments depends mainly on heat input during the welding processing.