As a solid-state welding technology, friction stir welding (FSW) can join dissimilar materials with good mechanical properties. In this paper, friction stir welding between 304L stainless steel and commercially pure copper plates with thicknesses of 3 mm was performed. A number of FSW experiments were carried out to obtain the optimum mechanical properties by adjusting the rotational speed to 1000 rpm and welding speed in the range of 14-112 mm/min and with an adjustable offset of the pin location with respect to the butt line. Microstructural analyses have been done to check the weld quality. Cross-sectioning of the welds for metallographic analysis in planes perpendicular to the welding direction and parallel to the weld crown was also performed. The mechanical properties of the welds were determined using a combination of conventional microhardness and tensile testing. From this investigation it is found that the offset of the pin is an essential factor in producing defect free welds in friction stir welding of copper and steel.