β-SiAlON hollow spheres were prepared by carbothermal reduction method, using coal fly ash (＜15 μm, 43-77 μm, and ＞100 μm) and active carbon in some proportion (20% less than theoretical addition, theoretical addition and 10% excess theoretical addition) as starting materials, putting into alumina crucible in high temperature nitriding furnace after well mixed, and holding at 1 300 ℃, 1 350 ℃, 1 400 ℃, 1 450 ℃, and 1 500 ℃ for 6 h. Effects of temperature, particle size of the microsphere, and addition of active carbon on the phase composition and microstructure of the nitridized products were studied by means of XRD and SEM. The results show that the nitridation reaction starts at 1 300 ℃; excess active carbon is necessary to form β-SiAlON hollow spheres, and particle size is the important parameter to form the hollow spheres nitridized products β-SiAlON; at 1 500 ℃, when the active carbon is 10% in excess, the β-SiAlON hollow spheres, which were prepared using coal fly ash with particle size＞100 μm, are featured with rough surface, high hollowness and low density.