Al 2O 3SiO 2 powders were prepared from a mixed solution of silica sol and aluminum nitrate by a coprecipitated process. The formation of mullite in coprecipitated powders was studied by quantitative X-ray diffraction and DTA analysis. The mullite nuclei were formed through the diffusion and reaction processes and their temperature dependence is determined by an apparent activation energy of 1080 ± 52 kJ/mol. The activation energy for the growth of mullite crystal obtained by isothermal analyses is close to the value by non-isothermal method. The activation energy of the crystal growth of mullite is affected by the pH condition, bulk chemical composition, and doping in the sample. A linear time-temperature-transformation diagram is presented.