The bimodal size SiC particulates (SiCp) reinforced magnesium matrix composites with different ratios of micron SiCp and nano SiCp (MSiCp:N-SiCp ¼ 14.5:0.5, 14:1, and 13.5:1.5) were prepared by semisolid stirring assisted ultrasonic vibration method. The AZ31B alloy and all as-cast SiCp/AZ31B composites were extruded at 350 C with the ratio of 12:1. Microstructural characterization of the extruded M14 þ N1 (MSiCp:N-SiCp ¼ 14:1) composite revealed the uniform distribution of bimodal size SiCp and significant grain refinement. Optical Microscopy(OM) observation showed that, compared with the M14.5 þ N0.5 (M-SiCp:N-SiCp ¼ 14.5:0.5) composite, there are more recrystallized grains in M14 þ N1 (M-SiCp:N-SiCp ¼ 14:1) and M13.5 þ N1.5 (M-SiCp:N-SiCp ¼ 13.5:1.5) composites, but in comparison to the M13.5 þ N1.5 composite, the average grain size of the M14 þ N1 composite is slightly decreased. The evaluation of mechanical properties indicated that the yield strength and ultimate tensile strength of the M14 þ N1 composite were obviously increased compared with other composites.