Material flow is the most important issue during friction stir welding (FSW) process because it directly influences the defect initiation, heat generation and microstructure of joint. Numerical simulation, based on computational fluid dynamics, is often used to investigate the material flow and heat generation in FSW. The interaction between welding tool and work piece is the most important and challenging phenomena for process modeling of FSW. In this paper, a novel tool / work piece interaction model is established. The material flow is driven by Coulomb’s frictional force, which is calculated based on the inhomogeneous interactive pressure on tool. A stagnant zone of material flow and a material vacant zone could be predicted with this model. The location of the material vacant zone corresponds to the wormhole defect appeared in experimental result. This research shows a potential method in predicting defects in FSW.