The hydrodynamic radii of colloidal silica particles in the aqueous solutions were measured under various conditions of pH and salt concentration by means of the dynamic light scattering technique. In the salt free solutions the observed hydrodynamic radius increased by 1 nm from 13 nm at pH below 8 to 14 nm at pH above 9 according to the ionization of silanol groups. At pH above 9, the radius decreased from 14 nm in salt free solutions to 13 nm at the salt concentration of 10(-2) M. These results suggest that the strong electrostatic electric field makes the hydration layer on the surface of a silica particle thicker.