Welding of comparatively thick (4 mm) C-Mn-Cr-Mo dual phase steel has been carried out by resistance spot welding process. Weldability of this steel has been studied by varying the electrode force and the primary welding parameters affecting the heat input such as the effective current and weld time. The influence of these welding parameters on the morphology, microhardness and the tensile shear strength of the weldment are investigated. Optimum welding parameters producing maximum joint strength are established as electrode force of 615 kg, effective current of 6 kA and weld time of 80 cycle. Weakening of weldment caused by excess tempering of martensite at the outer region of HAZ was not observed in the range of optimum welding conditions.