Tensile-shear test blanks were fabricated using high strength and low carbon steel sheets with three different strength levels by a resistance spot welding (RSW) procedure. The nugget sizes were changed by controlling the welding process parameters, resulting in the nugget sizes of 3√t, 4√t and 4.7√t, where t was the sheet thickness. Subsequently, tensile-shear static and fatigue tests were conducted to investigate the effects of strength levels of steels and nugget sizes on the mechanical properties of the welds. The tensile strength increased with increasing strength levels and nugget sizes. However, the fatigue strengths of the welds with the nugget sizes of 4√t and 4.7√t were nearly comparable irrespective of the strength levels of steels. That was because fatigue crack propagation life was dominant in the total fatigue life. In the welds with 3√t nugget size, the steel with higher strength level exhibited lower fatigue strength. It could be attributed to the lower bonding strength along the corona bond in the high strength steel.