Monodisperse colloidal silica particles were obtained in the ethanol medium in the presence of TEOS as silicon source and ammonia as catalyst. Transmission Electron Microscope and Laser Scattering Particle Analyzer were used to determine the average particle size. The effects of the amount of ammonia and water, the concentration of TEOS and the type of solvents on particle sizes were investigated. The formation mechanisms of the SiO2 particles under these conditions were also studied and discussed. The results showed that with increasing concentration of ammonia, the particle size increased due to the increase of [OH－]. The increase of the concentration of TEOS affected the diameter of the SiO2 particles remarkably. When the ratio of water to TEOS is much greater than 4 ultimately, the increase of water amount can have different consequence to the diameter of particles. In the propyl and butanol solvent system, monodispersed particles were not obtained due to agglomeration. （From : No.2, 2006 Glass & Enamel )