Friction stir processing was applied to 2024-T4 aluminum alloy plate under the condition of forced cooling. The microstructure of each zone (the base metal, the thermo-mechanically affected zone, the stir zone) and the effect of the advancing speed on the mechanical properties of the stir zone were studied. The results show that the grain size decreases gradually from the base metal, through the thermo-mechanically affected zone, to the stir zone. The microstructure in the stir zone is significantly refined and the grains are distributed uniformly with an average size of 1.9 μm. The base metal exhibits a large number of high-angle grain boundaries, while the thermo-mechanically affected zone has numerous low-angle grain boundaries. The distribution of misorientation angle in the stir zone presents a bimodal phenomenon. Hardness and tensile strength of the stir zone are increased with higher advancing speed