Abstract: A three-dimensional friction stir welding process model was developed based on fluid mechanics.The material transport in the welding process was regarded as a laminar,viscous and non-Newtonian liquid flows past a rotating cylinder pin.Results of the simulation show that the flow in the top weld is disordered because affected by the shoulder of the friction stir tool and the material under the shoulder moved around the pin for several circles.The flow in the bottom weld is more disciplinary,and just a little material close to the pin has experienced extreme deformation during the welding process.Material within the pin diameter passes the pin only on the retreating side in the rotation direction.The flow in the middle weld is affected by the pin and the shoulder and has both characters of the flow in the top and bottom weld.Material flow visualization experiments of "marker insert technology" was used to validate the simulated results.The experimental images agree well with the simulated results and the three-dimensional model can predict many of the trends observed in the experiments.