Abstract: The effective connection between 6061-T6 aluminum alloy and Q235 steel was realized by ultrasonic vibration enhanced friction stir welding (UVeFSW) process. The effects of ultrasonic energy on weld formation, joint microstructure, mechanical properties and welding load were investigated. The results show that the application of ultrasonic energy can significantly improve the weld surface forming and expand the width of aluminum / steel interface zone and weld nugget zone. Ultrasonic energy refines the grain structure of weld nugget zone and thermal mechanically affected zone, changes the fracture mechanism and cracking location of lap joint, improves the mechanical properties of joint. The variation of welding loading during welding was analyzed. It is found that the tool torque, the welding axial force and the spindle power decrease after applying ultrasonic energy.