The spherical silica particles with a narrow particle size distribution were prepared by colloid-aggregation method. This method involves combining a colloidal silica sol with a urea and formaldehyde, forming aggregates of colloidal silica and polymer. The experiments were carried out changing the amount of colloidal silica sol, urea, formaldehyde, water and other experimental conditions such as the reaction time, pH, kind of acid and solvent. As results, the average diameters of the silica microspheres prepared in various experimental conditions, after burned off the organic materials, were 3~15 μm from SEM analysis.