The size-dependence of growth rate of monodisperse silica particles was investigated by analyzing the competitive growth of a dispersion of silica spheres with a bimodal and trimodal size distribution, It was found that under the conditions that no new particles could be formed, particles with different size grew at the same rate, demonstrating that the growth was surface reaction limited process, Under conditions of formation of new particles, however, smaller particles grew faster than larger particles, indicating that the growth proceeded through diffusion limited process.