Fracture toughness is one of the parameters, which are used to estimate the fatigue life of resistance spot-welded (RSW) joints. A spot-welded pair is affected by the shear stress of the weld zone when it is exposed to tensile load. Repetitive loads reduce the fatigue life of the spot weld, and the material splits at the spot-welded region. This study investigates the effect of welding current, weld time and nucleus size ratios on the fracture toughness of RSW of galvanized DP450 steels having 1.0 mm thickness. The specimens were joined by spot welding at different welding currents and times. Welding processes were carried out using 3, 5, 7 and 9 kA welding current and 10, 20, 30 and 40 cycles (1 cycle = 0.02 s) weld time and the electrode pressure was fixed at 600 MPa. All series of specimens were exposed to tensile shear test in order to determine the fracture toughness. The fracture toughnesses for all series of RSW joints were calculated by using the formula given in the literature. The nugget diameters, core sizes and their heights were measured via an optical microscope. The Vickers microhardness measurement was carried out on the weld nugget, heat affected zone (HAZ) and base metal. Nucleus size ratios were calculated. The results of the study demonstrated that the fracture toughness of RSW depended not only on the nugget diameter D, but also sheet thickness t, tensile rupture force F, hardness H and nucleus size ratios h n /d n .