SiC　nanoparticles　reinforced　magnesium　matrix　composites　were　fabricated　by　ultrasonic　method.　The　AZ91　alloy　and　SiC　nanoparticles　with　the　average　diameter　of　50　nm　were　used　as　the　matrix　alloy　and　the　reinforcement,　respectively.　The　addition　of　nanoparticles　was　0.1%,　0.3%,　and　0.5%　(mass　fraction)　of　the　composites.　The　results　of　microstructural　evaluation　and　mechanical　properties　indicate　that　the　nanoparticles　can　be　dispersed　into　magnesium　alloys　efficiently　and　uniformly　with　the　aid　of　ultrasonic　vibration.　As　compared　with　the　matrix　alloys,　the　grains　of　composites　were　refined　and　the　mechanical　properties　of　composites　were　improved　significantly.　The　SEM　and　DSC　analyses　show　that　the　SiC　nanoparticles　can　act　as　the　heterogeneous　nucleation　of　alpha-Mg.　Also,　the　strengthening　mechanism　responsible　for　the　composites　reinforced　with　SiC　nanoparticles　was　discussed.