Ultrasonic welding of mild steel sheet (SPCC) and aluminum alloy sheet (A5052) was conducted using two di−erent weld tips of a knurled tip (K-tip) and a curved shape tip without knurled edges (C-tip). Behaviors of the weld tip and the welded materials during the weld process were analyzed using a high speed camera observation and an image correlation. The C-tip welding shows a completely di−erent displacement behavior from the K-tip welding; a relative motion between the tool tip and A5052 in contact with the tip predominantly occurs in the C-tip in contrast to a relative motion between A5052 and SPCC observed in the K-tip. The joint strength using the C-tip was higher than that using the K-tip under the weld conditions in this study. It is demonstrated that the relative motion behavior in the C-tip causes a macroscopic deformation of A5052 in the vibration direction. This phenomenon promotes growth of a bonded region, resulting in the increase of joint strength. It is surmised that Fe-Al intermetallic layer forms at the weld interface, and the joint strength consequently decreases for the longer weld time condition.