In this study, authors investigated the effect of horn tip geometry on the mechanical properties of a joint ultrasonically welded between a mild steel sheet and an aluminum alloy sheet. Two types of weld tips with the different contact face geometries were employed. One type of a tip has a cylindrical contact face without knurl. The other type of a tip has flat contact face with knurl. The main results obtained in this study are as follows. Authors successfully welded the mild steel sheet to the aluminum alloy sheet using both of the weld tips. However, the strength of the joint welded by the tip with cylindrical geometry was higher than that welded by the flat tip with knurl under the same welding condition, and the strength of the joint by the tip with cylindrical geometry was twice larger than that by the flat tip with knurl under the optimal welding condition. The strength of the joint increased with increasing the welding time and the clamping force, and showed the maximum under the reasonable welding condition. Excessively long welding time and large clamping force damaged the workpiece surface contacted the horn side tip, resulting in the decrease of the joint strength. It was possible to improve the joint strength using commercially pure aluminum foil with suitable thickness as insert metal.