10% (volume fraction) SiCp/Al-Mg composites with different Mg contents were successfully fabricated by semi-solid mechanical stirring technique under optimum processing conditions. Effects of Mg content on microstructure and mechanical properties were studied by scanning electron microscopy (SEM), X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The results indicate that SiC particles disperse homogeneously in Al-Mg matrix and interfacial reaction between Al matrix and SiC particles is effectively controlled. Distribution of SiCp reinforcement and interfacial bonding are improved by adding Mg. Additionally, the mechanical properties of composites are remarkably improved with the Mg content increasing.