The friction stir welding parameters were optimized to achieve the maximum hardness of the single phase brass joints. The results showed that the rotational and traverse speeds had the highest and lowest effect on the hardness, respectively. The empirical models predicted that the maximum hardness of 119 Hv can be reached by using a rotational speed of 700 rpm, a traverse sped of 100 mm/min, and an axial force of 1.5kN. The origin of the higher hardness value of the optimum condition compared to the other joint was due to the strengthening mechanisms of grain boundaries and dislocation densities. In addition, the Hall-Petch relationship between the hardness and grain size of the joints showed a deviation from the linear equation due to the formed substructures in the joints.