In this study, spherical fine pre-mullite particles were prepared by partial hydrolysis method to control the reaction rate difference of two alkoxides, and by mixed solvent method to control the particle shape. Based on the adaptation of Mie theory, the formation mechanism of pre-mullite particle in alkoxide-octanol-acetonitrile system has been investigated by measuring of the turbidity of solution, the number density and the size of the particles during the reaction. As the acetonitrile was added in the alkoxide-octanol solution, the solubility of the alkoxide decreased. A part of alkoxide was segregated from solution in the form of droplets, whereas the rest of the alkoxide existed as dissolved state in octanol. The droplets are stabilized with stabilizer (HPC). In the next step, hydrolysis and condensation reactions of the alkoxide droplets decrease the size of droplets and, at the same time, fine particles are formed from the alkoxide dissolved in octanol. Subsequently, the final particles are produced by aggregation between the large particles through the droplet form and the fine particles.