The effects of silica particle diameter on dispersion and aggregation behavior in water were analyzed, using alkoxide-derived silica powders with particle diameters of 8-260 nm. The present study focused on the relationships between the surface silanol structure and the interaction forces between solid surfaces in water. The surface silanol structure aml interaction between particles were determined using Fourier transform infrared spectroscopy. Fourier transform near-infrared spectroscopy, and atomic force microscopy. For relatively large particles (>30 nm in diameter), the surface silanols primarily were hydrogen-bonded silanols, and isolated silanols disappeared, The hydrogen-bonded silanols formed a hydrogen-bonded water layer on the particle surface; therefore, the additional hydration force was strong between these relatively large particles. In contrast, the surface density of isolated silanols increased as the particle diameter decreased to <30 nm, and the additional hydration force between ultrafine powders disappeared. The aggregation behaviors of alkoxide-derived silica powders were dependent on the hydration force, which was changed by the surface silanol structure.