In this article, effects of Friction stir welding tool rotational and traverse speeds were studied on the temperature distribution, material flow and formation of defects in the welding zone. Computational fluid dynamics method was used to simulate the process with commercial CFD Fluent 6.4 package. To enhance the accuracy of simulation in this Study, the welding line that is located between two workpieces, defined with pseudo melt behavior around the FSW pin tool. Simulation results showed that with increase of FSW tool rotational speed to linear speed, the material flow in front of tool became more and dimensions of the stir zone will be bigger. The calculation result also shows that the maximum temperature and stir of the material was occurred on the advancing side. The computed results showed that with incompetent heat generation, insufficient material flow caused around the pin and defects formed in weld root. The computed results were in good agreement with the experimental results of other researchers. Based on the welding parameters that used in this simulation, the maximum strain rate is predicted between -4(S-1) to +4(S-1) in the stir zone.