SS400/SUS304 steels lap joint was applied in various industries due to flexible andbeneficial properties of these dissimilar metals joint. Therefore, an investigation for optimization ofa gas metal arc welding (GMAW) for producing the dissimilar metal lap joint should be conductedfor advancing the manufacturing industries. This research applied GMAW with various currents andspeeds to weld SS400/SUS304 lap joint and studied the relationship between the parameters andresuted joint properties. The experiment showed that an increase in the wedling current and a decreasein the welding speed affected to increase the fracture strength, the displacement of the lap joint, andthe joint hardness due to high combination of the materials at the joint interface. The uncombinedmaterials at the carbon steel/weld metal interface had the different hardness and resulted to initiatethe crack that then was propergated until the joint was broken. The weld metal showed the formationof the finer and smaller dendrite structure with increasing the welding current and decreasing thewelding speed.