A high intensity ultrasonic assisted casting method was used to fabricate SiC nanoparticles reinforced magnesium matrix nanocomposites (n-SiCp/AZ91D). The microstructures and mechanical properties of the nanocomposites were investigated. The results show that n-SiCp are well dispersed in the matrix and the grain size was refined. A HRTEM study of the interface between n-SiCp and the matrix suggests that SiC bonds well with matrix without forming an intermediate phase. With the lower addition of n-SiCp, the mechanical properties of nanocomposites are greatly improved. As compared to an unreinforced magnesium alloy matrix, the tensile and yield strength were improved by 43.6% and 117% respectively.