The　SiC　nanocomposite　Mg-2(Si0.4Sn0.6)Sb-0.015/(y)SiC　(0　＜=　y　＜=　0.01)　with　Sb　doping　were　prepared　using　the　vacuum　induction　levitation　melting　technique　in　conjunction　with　mechanical　alloying　and　spark　plasma　sintering　process.　The　microstructure　and　thermoelectric　properties　were　systematically　investigated.　The　hierarchical　nanostructures　for　the　samples　are　composed　of　point　defects,　SiC　nanocomposite　particles,　coherent　in-situ　nanophases,　and　grains.　The　SiC　nanocomposite　cause　a　minor　reduction　in　electrical　performance,　and　the　constructed　hierarchical　nanostructures　significantly　decreased　the　lattice　thermal　conductivities.　The　highest　figure　of　merit　value　of　1.55　is　obtained　at　643　K　for　the　y　=　0.0075　SiC　nanocomposite　sample.