Wettability and the adhesion force between two silica particles were measured as a function of relative humidity. The wettability of silica particles was controlled by rehydroxylation and modification by hexamethyldisilazane and evaluated by a preferential dispersion test, heat of immersion, and water adsorption. The adhesion force between two particles was measured by atomic force microscopy using a "colloidal technique". The wettability varied from hydrophilic to hydrophobic at trimethylsilyl density 1.0 nm(-2) as modified groups shielded the residual silanol groups and hindered the formation of a capillary bridge between the two particles. This "hindrance effect" is thought to result in the absence of a critical increase in adhesion force at high relative humidity and also the reduction of surface energy at low humidity where hydrogen bonding dominated the adhesion force.