The purpose of this study is to investigate the effect of resistance spot welding (RSW) parameters on nugget size and ultimate strength of Magnesium alloy sheets AZ61 under tensile-shear test. In this study microstructural examination and hardness measurements were carried out on the welded samples. The results show that the weld nugget zone is divided into two separate parts: the equiaxed dendritic zone (EDZ) perched at the center of the weld nugget and the columnar dendritic zone (CDZ) situated around the fine-grained zone having it surrounded. The effect of the following three parameters: electric current, welding time and electrode force on the dimensions of the weld nugget and the welded ultimate strength is investigated. The response surface method (RSM) is employed to examine the effects of welding parameters and to attain optimum parameters. The analysis of variance (ANOVA) results of RSM model shows that however the tensile-shear strength and nugget size are improved with increasing the welding current and welding time, the welding current is the most influential parameter. In addition, the optimal values for the welding parameters are calculated to achieve the maximum nugget size and the ultimate strength of welded joint. Finally, a regression model is proposed in order to predict the peak load and the nugget size as function of the mentioned welding parameters. Manuscript profile