The widespread application of resistance spot welding in automotive body assemblies has made it mandatory for automotive manufacturers to examine the quality of the welding. A variety of destructive and non-destructive tests are applied and the weld nugget size is examined for inspection for weld quality. Because of the wide variety of welding parameters used in the spot welding, a welding diagram which gives the optimum weldability range for the welded material is required and extensively used. Due to the different chemical, physical and mechanical properties of different materials in dissimilar joints, it is difficult to evaluate the quality of the weld. In this study, the DP600/TRIP800 dissimilar steel couple was joined using resistance spot welding by combining various welding parameters including welding current, welding time and electrode force. The tensile shear test was applied for determining relationship between welding parameters and mechanical properties of weldment. For quality inspection, the weld nugget size and microstructure of welded sample was examined metallographically and the welding diagram dedicated to the optimum weldability range was also developed for dissimilar joints.