Abstract: SiC/Al-18Si composites with different SiC content by mass were in-situ synthesized by cold pressing and vacuum sintering, using aluminium powders, silicon powders, and graphite powders as raw materials. The phase composition and microstructures of the aluminum matrix composites were characterized by X-ray diffractometer (XRD), scanning electron microscope (SEM), and energy disperse spectroscope (EDS). The effect of SiC on the microstructure, flexural strength, and microhardness of the composites were analyzed, and the variation in mechanical properties of the composites was discussed. In the results, the matrix phase of the composites is Al phase and the second phases are Si and SiC phases. The fine SiC particles by in-situ synthesis with the size ranging from 0.2 to 2.8 μm can be dispersed in Al matrix, showing the multi-scale characteristics of micron and submicron. With the increase in SiC content by mass, the microhardness of SiC/Al-18Si composites increases; meanwhile, the average sizes of SiC particles increase from 0.81 to 1.13 μm, but the SiC particles are still uniformly distributed in Al matrix, making the microhardness of the SiC/Al-18Si composites much higher than that of Al-18Si. When the mass fraction of SiC is 30%, the microhardness of the SiC/Al-18Si composites is the highest (HV 134), which is 88% higher than that of Al-18Si.