Welding of both carbon steel (AISI 1020) and Monel 400 by using friction stir welding is investigated in this article. The objective is to determine the effects of welding process variations in traverse and rotating speeds on joint quality. The traverse speed was adjustable among 50 and 100 mm/min, and the rotating speed was set between 200 and 400 rpm. It was found that 200 rpm rotational speed and 50 mm/min traversal speed produced best possible weld quality. And mainly 200 rpm rotational speed and 50 mm/min traversal speed produced the best possible weld quality. On the other hand, high traversal speeds of more than 200 mm/min produced flaws that resembled grooves. Three successive joints were accomplished (Monel400- Monel 400), (Monel 400- AISI carbon steel butt joint) and (Monel 400- AISI carbon steel lap joint). These welds' mechanical characteristics were thoroughly assessed using room-temperature. Tensile, impact, bending, and microhardness tests were held, The results of tensile test for three joints were 585MPa ,450MPa and 460MPa respectively, where for the hardness results were about 93 Kg/mm2 and 81 Kg/mm2 for Similar and Dissimilar joints. Metallurgical and mechanical tests showed that the joints had outstanding hardness, impact resistance. Scanning electron microscopy (SEM) and optical microscopy were used to analyze the alterations in the welds' microstructure.