The engineering industry uses magnesium as it is a low density to lightweight ratio material and able to replace the heavier material. Friction stir processing is an applicable method to modify the structural properties of the workpiece. H13 steel tools are produced into several tool parameters with different shoulder diameters to pin diameters (D/d) ratios. A fixed machining parameter of 1040 rpm for spindle speed and 17 mm/min for traverse speed was used throughout this study. Contact between the tool and workpiece produces frictional heat that softens the material. By creating magnesium alloys into metal matrix composites (MMC), microsized aluminum oxide powder (Al2O3) was reinforced during FSP to enhance the mechanical properties of the magnesium alloy AZ91A. The aim of this study is to analyze and obtain the optimal tool parameter to process Mg-Micro Al2O3. The microstructure of FSPed Mg-Micro Al2O3 was observed using a light microscope, specifically on the grain size. The hardness test was done utilizing the Rockwell Hardness Tester to validate the changes in the hardness. The shoulder diameter of 12 mm was found to be the most suitable parameter for processing Mg-Micro Al2O3 as it produced fewer defects and finer grain size.