Magnesium matrix composites reinforced with nano-sized SiC particles (n-SiCp/AZ91D) were fabricated by high intensity ultrasonic assisted casting. The microstructure of the nanocomposites was investigated by optical microscopy, scanning electronic microscopy (SEM) methods and room-temperature mechanical properties were measured using a universal electronic material tester. The results showed that n-SiCp addition can refine the grains, and the distribution of n-SiCp in magnesium alloy melts is significantly improved by ultrasonic processing. Compared with the unreinforced AZ91D matrix, mechanical properties of the nanocomposites including tensile and yield strengths and hardness are remarkably improved; in particular the yield strength is increased by 57% after gravity permanent mould casting.