Metal matrix composites (MMCs) have emerged as an important class of materials and are increasingly utilized in various engineering applications which require materials offering a combination of light weight with considerably enhanced mechanical and physical properties such as strength, toughness, stiffness and resistance to high temperature. However, the applications of MMCs are limited by the poor machinability of the materials, which is a result of their highly abrasive nature that causes excessive tool wear in the machining processes. It has been found that the machinability of MMCs is dependent on the percentage of the reinforcement of the MMCs. It is wise to design a MMC having the reinforcement percentage that is good to both the mechanical proper-ties and machinability of the MMC. This is achievable if the mechanical properties and machinability of the MMC in relation to the percentage of reinforcement is known. In this study, Mg-Al-SiC MMC specimens of a wide range of different particle size and percentage reinforcement are fabricated using powder metallurgy, and are tested for the mechanical properties and machinability. The mechanical properties measured include the yield strength, ultimate tensile strength and ductility. The machinability tested is the tool wear characteristics. A map of mechanical properties and machinability of Mg-Al-SiC MMC in relation to the reinforcement is developed from the results. It is shown surprisingly in the map that good mechanical properties of the MMC do not necessary mean poor machinability. Although in general both the tool wear rate and the mechanical properties increase with the increase of the particle size and percentage of the reinforcement, the relationships are non-linear, which result in a combination of the reinforcement particle size and percentage giving the MMC both high mechanical properties and good machinability.