Abstract: Friction stir welding as a new method of joining is getting more popular whereas the statistical approach as an advanced method of quality management is widely used. Here, a full factorial design 22 (two factors at two levels) with replicates was conducted to investigate the influence of the factors rotational speed (900 and 1200 rpm) and the welding speed (40 and 60 mm min-1) in the Friction Stir Lap Welding (FSLW) process for the purpose of UTS-AS, UTS-RS, grain size-WZ, grain size-HAZ and EPT. The study used plates of Al-alloys AA6061-T6. The factors rotational speed and welding speed were significant and an increase in the welding speed and a decrease in the rotational speed contributed to all the responses in FSLW. Effect of interaction was not significant for all responses such as UTS-AS, grain size-WZ, grain size-HAZ and EPT and the contribution of this effect to the UTS-RS were positive, but less than 18%. The response surface analysis and table of ANOVA indicate that the most effective level of factors for FSLW process in this study was achieved using a 900 rpm and 60 mm min-1 solution of rotational speed and a welding speed, respectively.