Colloidal silica which has high surface area and excellent surface properties are chemically stable inorganic materials and used for various applications in industry. Therefore the effects of the experimental conditions such as the content of silica in aqueous solution, pH of solution and reaction temperature on the formation of silica sol were investigated for the preparation process of colloidal silica sol from sodium silicate solution in this study. The properties of sodium silicate solution and silicate solution removed sodium ion by ion exchange was characterized by yellow silicomolydate method. The SiO2 content of 2% in sodium silicate solution was proper to oxidize sodium silicate with sulfuric acid. After the removal of sodium ions in sodium silicate solution, the pH of silicate solution had to be controlled above 9 for a stable silicate solution. The silica sol, which size is about 10 nm, could be prepared by heating the mixed solution of sodium silicate and silicate solution removed sodium ions at pH 10 and 80°C. And the silica sol grew into about 50 nm as silicate solution was added to silica sol solution.