SiC nanoparticles reinforced magnesium matrix composite was fabricated by ultrasonic vibration assisted squeeze casting. Since ultrasonic device could meet the use requirements according to theoretic calculation, uniform dispersion of SiC nanoparticles was expected to achieve. The grains of the composite were refined compared with the AZ91 alloy, which was related to the increase of nucleation sites during solidification and Zenner pinning effect caused by SiC nanoparticles. With increasing the ultrasonic power, grain size of the composite changed no obviously while the morphology of β-Mg17Al12 phase was significantly affected. The ultimate tensile strength, yield strength, and elongation to fracture of the composites fabricated under different ultrasonic powers were simultaneously improved compared with the AZ91 alloy. The increase of yield strength could be attributed to Hall–Petch strengthening and Orowan strengthening for the present composites. Theoretical value of the yield strength obtained by the square root method was close to the experimental value.
