The effects of reaction parameters such as [NH₃],r(H₂O/Si molar ratio) and temperature on hydrolysis, condensation and final silica particles, and the effects of electrolyte addition on the change of final silica particle sizes in the solgel process were investigated. In our experimental conditions, the increment of [NH₃] and r accelerates the hydrolysis and condensation and increases the particle size. But particle size decreases when r=20 as compared to when r=10. The activation energies of hydrolysis and condensation are 5.78 and 3.34 kcal/ mol, respectively, and the increase of the temperature decreases the particle size. The addition of salt to solutions does not affect the reaction rates, but increases the final silica particle sizes. The increase of the electrolyte concentration and the number of counter ions in electric double layer increase the final silica particle sizes due to the mobility of the counter ions in LiCl, NaCl and KCl. These results are quite compatible with DLVO theory in which the increase of κ(Debye-Huckel constant) decreases primary particle size and increases final particle size. Correlating primary particle sizes to final particle sizes obtained by the aggregation growth mechanisn shows that the final partcles grow through the aggregation of primary particles.