Abstract: The four kinds of 5 μm SiC_P/Mg alloy (AZ91) magnesium matrix composite with the volume fraction of 2%, 5%, 10% and 15%, respectively, were fabricated by stir casting. After the solution treatment, forging and hot extrusion, the effect of SiC_P on the microstructure and mechanical properties of AZ91 matrix were investigated by compared with the monolithic AZ91 alloy. The results show that the grain size of SiC_P/AZ91 depends on the volume fraction of SiC_P. The average grain size of composite matrix decreases as the volume fraction increased from 0% to 10%, the grain size increases as SiC_P volume fraction continuing increased to 15%. The addition of SiC_P can improve the yield strength and modulus of AZ91 significantly, and they increase with the volume fraction of SiC_P. The strengthening effects of SiC_P on AZ91 matrix are mainly attributed to dislocation strengthening mechanism, grain refinement and load transfer effect, the grain refinement plays a main role on improving the yield strength.