This research aimed to study the effect of gas metal arc welding parameters on 1.4003 ferritic stainless steel and SS400 carbon steel. There are three parameters for the experiment investigating the microstructure of the joint, Inspection of weld properties. It consists of a tensile test and a hardness test. The result is as follows. The gas metal arc welding could successfully produce the sound weld of 1.4003 ferritic stainless steel and SS400 carbon steel. An increase in welding speed affected to decrease in the tensile strength of the welds but did not affect to change the hardness of the welds. The study found that. The optimized welding parameter in this study that showed the ultimate tensile strength was welding wire ER308 LSI, welding current 120A, and welding speed 150 mm/min.