Abstract: The behaviour of kyanite decomposing into mullite and SiO₂ in different temperatures, different medium conditions and different grain sizes was studied. The experiments showed that the decrease of grain sizes can largely decrease the decomposing temperature and increase the decomposing speed. When the kyanite is contacting with a melt which produced in decomposing process, the decomposing temperature can be significantly dicreased and the decomposing speed can be largely increased.