Abstract: Three-dimensional Monte Carlo model was applied to simulate the grain growth process of the stirring zone during friction stir welding of AA2024-T3 aluminum alloy. The relationship between the Monte Carlo steps and the temperature history and time history of the stirring zone was established. The variation of grain size in the stirring zone at different rotation speeds of stirring head was simulated and verified by the experimental values under the same welding conditions. The results show that the three-dimensional Monte Carlo model could reflect the changes of grain size in the stirring zone at different rotation speeds of stirring head well. The simulating predicted values of the average grain size were in good agreement with the experimental values. The average grain size in the stirring zone increased with the increase of the rotation speed of stirring head. The average grain size in the advancing side of the stirring head was slightly larger than that in the retreating side and the average grain size of the upper surface of the center seam in the stirring zone was the largest when the rotation speed of stirring head was the same.