Friction stir processing (FSP) is used to fabricate nano Al2O3 reinforced 6061 Al surface nanocomposite with the aim to get defect-free surface nanocomposite with single-pass FSP. In this research work, the impact of tool shoulder geometry, rotational and traverse speeds of the tool on the microstructure, material flow, and microhardness of the fabricated surface nanocomposite was studied. At a particular combination of the process parameters, a uniform mixture of the aluminum matrix material and Al2O3 nano reinforcement particles were mixed well and exhibited good mechanical and hardness properties. The fabricated surface nanocomposite showed higher microhardness than the as- received 6061 Al alloy. The average microhardness of the composite was achieved as similar to 132 Hv with 24 mm shoulder diameter of the tool. The results show that FSP is an effective way to produce Al/Al2O3 surface nanocomposite with excellent microstructural and mechanical properties.