Abstract The colloidal silica via sodium silicate was modified by adding the Al ion in the surface growth process of colloidal silica. The particle sizes of modified colloidal silica were decreased by increasing the seed concentration. But the particle size of colloidal silica was slight increased as seeds concentration was 30 wt.%. Aluminum ions on the surface of modified colloidal silica was identified by the measurement of EDS. The iso-electrical point (IEP) of colloidal silica was changed from pH ∼2 without modification to disappear over the whole pH range after modified with Al ion.