Friction stir processing (FSP) is a post-processing method that locally manipulates microstructure by imparting a high level of energy in the solid state giving rise to improved mechanical properties. FSP was applied to 25.4 mm thick, sand casting A206 aluminum workpiece under different parameters. Effects of FSP on microstructure evolution will be described and discussed. Specifically, through optical microscopy and SEM measurements the attained refinement in the stirred zone will be reviewed; porosity is significantly reduced and second phase particles are fragmented. Thermal conditions of the FSP zone have been measured as well as studying the effect of the temperature gradients on the resultant structure.