Abstract: To identify the main cause of fatigue failures of welded joints of 6N01-T5 aluminum alloy extrusions, fatigue tests were carried out upon 6N01-T5 aluminum alloy extrusions and their weld joints, and S-N curves and conditional fatigue limits of them were obtained.Microstructure and mechanical properties of the welded joints were investigated.The fatigue fractures of the joints were analyzed and thereby the characteristics of them were obtained.The results show that the microscopic structure of the joints is α-Al and pseudo eutectic made up of α-Al and Mg₂Si;the defects of the joints are mainly gas cavities;there is a soften zone between HAZ and base metal which lead to fracture under dead-load but hardly to fracture failure;the main influence factors of fatigue performance are defects on the surface of weld and the surface condition of the weldment.