Friction stir welding (FSW) process is a promising solid-state joining process with the potential to join low-melting-point material, particularly, aluminum alloy. The most attractive reason for joining aluminum alloy with this process is the avoidance of the solidification defects formed by conventional fusion welding processes. In this research article, an attempt has been made to develop an empirical relationship between FSW variables and Micro Hardness. A factorial design was used by considering three factor and eight trials, which enables to quantify the direct and interactive effect of three numeric factors, that is, tool rotational speed, welding speed, and shoulder diameter on the Micro Hardness. The developed relationship is useful for prediction of Micro Hardness in friction stir welded AA6061 aluminum alloy joints at 95% confidence level. It will also be helpful for selection of process variable to obtain the desired strength of the joint. Furthermore, the optimized capabilities in design- the expert software was used to numerically optimize the input parameters.