Abstract: (O'+β)-Sialon/mullite composite materials were synthesized by in-situ carbothermal reduction-nitridation process, with fly ash and carbon black as raw materials. The influences of carbon content on phase composition and microstructure of the synthesized materials were investigated by XRD and SEM, and the formation process of the materials was analyzed. The results indicated that increasing carbon content benefits the formation of O'-Sialon and β-Sialon. The (O'+β)-Sialon/mullite composite materials can be synthesized at 1350°C for 6 h, where the mass ratio of fly ash to carbon black in samples are 100/42 and 100/56 separately. The O'-Sialon and β-Sialon in the synthesized materials are in particle shape with an average grain size about 1 μm. The formation process of (O'+β)-Sialon/mullite composite materials consists of the formation of O'-Sialon and β-Sialon in addition to the conversion process of O'-Sialon to β-Sialon.