β-Sialon powder was synthesized by in-situ carbothermal reduction-nitridation process, with fly ash and carbon black as raw materials. The influence of raw materials composition on synthesis process was investigated, and the phase composition and microstructure of the synthesized products were characterized by X-ray diffraction and scanning electronic microscope. The carbothermal reduction-nitridation reaction process was also discussed. It was found that increasing carbon content in a sample could promote the decomposition of mullite in fly ash and the formation of β-Sialon. The β-Sialon could be synthesized at 1550°C for 6h by heating the sample with the mass ratio of fly ash to carbon black of 100:56. The β-Sialon as-received in this study existed as granular with an average particle size of about 2μm. The carbothermal reduction-nitridation reaction process consisted of the nitridation processes of mullite, SiO2 and Al2O3 in fly ash as well as the conversion process of X-Sialon to β-Sialon.