Abstract: Microstructure and fatigue properties of welded joints of two 6005A alloy were investigated by scanning electron microscopy(SEM), transmission electron microscopy(TEM), tensile test and fatigue test. Moreover, the main cause of the fatigue crack initiation of welded joint of 6005A alloy was revealed. The results show that Cu can clearly improve the tensile and fatigue properties of commercial 6005A alloy. The tensile strength, elongation, fatigue strength of welded joint of self-made 6005A aluminum alloy are 220 MPa, 12% and 106 MPa respectively. The size and the distributtion of precipitation phase Mg₂Si in welding heat affected zone affect the fatigue of the 6005A joint significantly. So, it improves the composition control in welding heat affected zone, the segregated degree on Mg₂Si phase away from weld softening zone and the fatigue properties of welded joint.