Abstract Two-stage refilled friction stir spot welding (TFSSW) is a novel refilling process, based on conventional FSSW. This process is done in two steps and with two independent tools, conventional and refilling tool. In this paper, TFSSW is studied, using Al 1100 sheets which were 2 mm in thickness, to evaluate effect of welding parameters on joint strength. Tool rotational speed is the most important parameter that affects welding strength followed by refilling tool shoulder diameter. Application of lower tool rotational speed and bigger refilling tool shoulder diameter leads to stronger joints. Dwell time is the negligible parameter, because it has almost no effect on joint strength. Welding parameters for strongest joint, with 6.96 kN tensile shear strength, were tool rotational speed of 1000 rpm, conventional tool plunge depth of 3.8 mm, total dwell time of 6 s and refilling tool shoulder diameter of 14 mm.