Ultrasonic welding between SS400 mild steel sheet and aluminum alloy A5052 sheet containing magnesium was conducted. In this study, authors investigated the influence of ultrasonic welding condition on the mechanical properties and the interface microstructure of a joint, and examined the effect of an insert metal to improve the joint strength. The main results obtained in this study are as follows. It was possible to ultrasonically weld SS400 mild steel sheet and A5052 sheet. When the clamping force was varied keeping the welding time constant for 1s, the joint strength showed the maximum at the clamping force of 588N and it decreased with increasing the clamping force because the frictional action at the interface decreased with it. When the clamping force was kept constant at 588N and the welding time was varied, the joint strength reached the maximum at the welding time of 2.5s, followed by decreasing in the joint strength welded at 3s welding time due to the formation of Fe2Al5 intermetallic compound at the interface. Using insert metal of commercially pure aluminum successfully improved the joint strength and the strength of the joint welded at 3s welding time was about three times larger than that without the insert metal.