Monodisperse silica particles were prepared by hydrolysis and condensation of tetraethylorthosilicate (TEOS) at various impeller speeds up to 600 rpm to study the effects of impeller speed on particle size distribution. Ammonia-catalyzed reactions of TEOS were carried out in an ethanol-water solution at a water concentration of 11.0 kmol/m(3), ammonia concentrations of 0.5 and 1.0 kmol/m(3), and TEOS concentrations of 0.2 and 0.4 kmol/m(3) in the presence or absence of an electrolyte (KCl). When the final particles prepared were small, the particle size hardly depended on impeller speed; but, as the size or final particles increased, the particle size more strongly depended on impeller speed, and an increase in impeller speed increased the size of final particles. This impeller speed dependence is considered to be caused by sheer flocculation, which dominates over Brownian flocculation in the case of large particles, When particle size was increased by the addition of KCl, agitation at high impeller speeds brought about excessive coagulation, resulting in generation of hard aggregates and disappearance of spherical particles.