Abstract: By means of SEM,TEM,laser scattered particle analyzer,etc,,the forma-tion mechanism of ultrafine SiO_2 particles behaved by the controlled hydrolysis of metalalkoxide was studied.By controlling concentration of reactants and catalyst,the concentra-tion of silicic acid monomer was set as high as possible,but just below the critical supersatu-ration.Under these conditions,monodispersed and spherical ultrafine SiO_2 powder could beprepared.The average particle size was from 60 to 500 nm. It was demonstrated that the re-action temperature,feeding speed,ageing time affected the particle size and distribution.