Friction stir processing (FSP) is a new solid state processing technique for microstructural modification in metallic materials. FSP has been applied to cast aluminum alloy A356 plates to modify the microstructure to enhance mechanical properties. FSP broke up and dispersed the coarse acicular Si particles creating a uniform distribution of Si particles in the aluminum matrix. Further, FSP healed the casting porosity. These microstructural changes led to a significant improvement in both strength and ductility. Generally, high tool rotation rate is beneficial to break coarse Si particles, heal the casting porosity, and consequently increase strength. For a standard pin, maximum strength was achieved at a tool rotation rate of 900 rpm for a constant traverse speed of 8 ipm. A tri-flute pin design produced an optimum combination of strength And ductility compared to the standard and cone-shaped pins.