Abstract: Friction stir welding (FSW) is a new solid-stats joining technique that has been invented recently. This process produces low-distortion, high-quality, low-cost welds on aluminum alloys even for those difficult to the traditional fusion welding, In this paper,a three-dimensional finite element modeling of FSW process is presented. The modeling effort includes a de-coupled heat transfer and a subsequent thermo-mechanical analysis. The temperature fields during welding, the residual stress distribution and distortion of the workpiece after FSW process are studied for aluminum alloy 6061-T6. The heat transfer analysis shows that the maximum temperature during FSW is not higher than 80% of the melting point. It provides possibility for predicting the microstructure of the joint made by FSW technique. The thermo-mechanical analysis shows that the residual stress and distortion after FSW are much lower than that after traditional fusion welding. The maximum residual stress is only about 25%-30% of the yield stress of base metal. The results from the modeling are consistent with the available experimental data and trends.