Abstract: This work used ultrasonic-assisted friction stir lap welding to join dissimilar Al/Ti alloys. The effect of ultrasonic vibration on joint formation and microstructure was mainly studied. Results show that void appeared inside the joint when the ultrasonic vibration was applied on the Al sheet. Defect-free joint was obtained when the ultrasonic vibration was applied on the Ti sheet. Ultrasonic vibration could enhance the diffusion at the Al/Ti interface, which was beneficial for joint mechanical properties. The thickness of the diffusion layer was rather thin at the rotating speed of 300 r/min. An intermetallic compound with a thickness of 1 µm was obtained at the rotating speed of 500 r/min. The application of ultrasonic could significantly increase the failure load of the joint.