<p>The use of thin ultra-high tensile strength steel (UHSS) sheets for automobiles to reduce their weight is an effective method to decrease greenhouse gas emissions, and they are adopted to chassis parts where the sufficient fatigue life of lap-fillet arc-welded joints is required. Generally, the fatigue life of welded joints does not necessarily increase with steel sheet strength, since fatigue behavior is primarily affected by weld geometry and local stress concentration at weld toe. In this study, the effect of Cr contents in welding wires on the shape and hardness of weld metals and fatigue life was investigated by using lap-fillet gas metal arc welding (GMAW) joints of 1.2-GPa cold-rolled UHSS sheets. Four types of solid wires for GMAW with wire Cr contents from 1 to 15% were applied. The fatigue life of joints welded with higher-Cr wires tended to be longer than that of 1% Cr wire. Weld metal hardness also increased with increasing wire Cr content. The shape of the weld metal varied depending on wire Cr content and welding condition. In particular, high-Cr welds with flatter weld metal shapes, characterized by lower height, greater width, and smaller toe angles, showed improved fatigue life. This improvement was attributed to reduced stress concentration at the weld toe and increased weld metal hardness. For forming flat weld metal, it was effective to change the wire tip aiming position from the root to the bottom sheet side. Consequently, controlling both the shape and hardness of the weld metal by selecting welding wire composition and wire aiming position is a practical method to improve fatigue life.</p>