Abstract: In this study, we evaluated the microstructure and mechanical properties of friction stir welded composite base aluminum with 20% sio2 particles. One of the most effective parameters on microstructure and mechanical properties in the welding zone of friction stir welding (FSW) is Rotational velocity and forward motion of the non-consumable tool which the major factors affecting the rate of input heat in the process. in this research, effect of changes in rotation speed and progress of welding tool on the microstructure and mechanical properties of aluminum base composite reinforced with 20% sio2 particles was prepared by stir casting has been studied. Finally, it was found that reducing the welding step in this procedure will cause defects in the weld. Experimental results were confirmed optimum rotation speed and forward velocity of the welding tool.