The carbon black and graphite were used as carbon sources to fabricate reaction-bonded SiC, and the total content of carbon sources was 10% (mass fraction, the same below). The effects of different ratios of carbon black and graphite on properties of reaction-bonded SiC were investigated. The results show that the reaction-bonded SiC with 4% carbon black and 6% graphite has good mechanical properties, the flexural strength and fracture toughness reach to 251.7 MPa and 4.29 MPa·m1/2. The mineral composition of reaction-bonded SiC was measured by XRD, the Rietveld simulation results of XRD patterns were adopted to measure phase content. The free Si content in reaction-bonded SiC with 4% carbon black and 6% graphite is 24.44% (mass fraction). The free Si content in reaction-bonded SiC with 10% graphite reaches to 28.57% (mass fraction), which is higher than that of the former. The decrease of free Si contributes to the improvement of mechanical properties of reaction-bonded SiC.