Abstract Mg matrix nano-composites (MMNCs), reinforced with 1wt% SiC nanoparticles, were cast by ultrasonic cavitation-based dispersion methods. Microstructural analyses of as cast specimens were conducted to characterize the grain size, shape and distribution, SiC nanoparticle size and nanoparticle-matrix interface. Average grain size for the composite specimens was 72μm compared to 181μm for pure Mg samples. The average SiC nanoparticle size was 66nm. TEM studies showed good local dispersion of SiC nanoparticles, with only a few small largely spaced agglomerates. HRTEM showed a clean interface between SiC nanoparticles and the Mg matrix. The yield strength of Mg-1wt% SiC nanocomposites was 67MPa which showed improvement from 47MPa for the pure Mg samples. This extra strengthening is due to Orowan and Hall–Petch effects.