The effects of additional Mg content and the size of the reinforcement phase on the mechanical properties of 20 vol.% SiCp/AC8A composites fabricated by pressureless metal infiltration process were investigated. The hardness of SiCp/AC8A composites increased gradually with an increase in the additive Mg content, while the bending strength of SiCp/AC8A composites increased with an increase in additive Mg content up to 5%. However, this decreased when the level of additive Mg content was greater than 5%. The increase in hardness by additive Mg content is due to the formation of hard reaction products such as Mg2Si and AlN during the process, whereas the decrease in bending strength when Mg content was above 5% is due to the formation of coarse precipitates by excessive Mg reaction and an increase in the porosity level. The hardness and strength of the composites increased with a decrease in the size of SiC particles. It was found that the composites with smaller particles enhanced the interfacial bonding more than those with bigger particles from fractography of the composites.