Nanoscaled cristobalite and alpha-Al2O3 powders were used as the starting materials for synthesizing mullite by solid-state reaction. The thermal reaction of the cristobalite with alpha-Al2O3 during the thermal treatment was examined. Cristobalite powder with a D-50 value of 430 nm was adopted to mix with alpha-Al2O3 powders with a D-50 values of 230, 310, and 400 nm in a stoichiometric composition of 3Al(2)O(3)center dot 2SiO(2) (71.8 wt% alpha-Al2O3 and 28.2 wt% SiO2). Samples for thermal reaction were prepared using uniaxial pressed from the three mixtures that showed various particle number ratios of SiO2/Al2O3 due to the different particle sizes of alpha-Al2O3. Examinations were performed by differential thermal analysis, X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy techniques. The results showed that cristobalite particles amorphized during the thermal treatment, and then reacted with the alpha-Al2O3 particle to form mullite via nucleation and growth. The amorphization temperature can be reduced by using finer-sized alpha-Al2O3 powders, thus leading to a lower temperature for mullite formation. Mullite crystals with a multidomain structure were observed in the alpha-Al2O3 particle matrixes. The crystal orientation of the mullite was controlled by the alpha-Al2O3 matrix, that is, [001] alpha-Al2O3 -> [001] mullite. These results indicate that the amorphization of cristobalite may trigger the reaction of SiO2 with alpha-Al2O3, initiating the nucleation of mullite. The alpha-Al2O3 particles act as the hosts for mullite formation and determine the size of the mullite particles.