Two kinds of gradient composites, whose compositions were Al-20%Mg2Si-5%Si (sample-1) and Al-14.9%Mg2Si-10.3%Si-11.8% MgAl2O4 (sample-2), were fabricated based on the electromagnetic separation under the high frequency magnetic field. The microcosmic structures of two composites were analyzed and the effect of the addition of SiO2 on the microcosmic structure and the mechanical property were discussed. It is found that the particles MgAl2O4 accumulated in periphery of specimen of sample-2, which come from the diffusion reaction between SiO2 and aluminum and magnesium in the melt and was proved by the result of X-ray analysis. Besides the MgAl2O4, the primary phases of Mg2Si, Si also emerged in periphery of specimens. At last, the hardness was given along the radial direction. The result shows that the hardness of the two materials exhibit graded distribution in radial direction and the hardness of sample-2 is higher than that of sample-1 due to the formation of MgAl2O4 particles. Both the two materials meet the requirements of gradient composite material whose outer area is in high intensity and center area is in flexile.