The aggregates properties of amphipathic molecules formed at solid-liquid interfaces are influenced by the environmental conditions: solution concentration, temperature, kind of counterion, etc. The effect of the counterions on the adsorbed aggregates was investigated by atomic force microscopy in surface force mode and in surface imaging mode for alkyltrimethylammonium salts (hexadecyltrimethylammonium bromide; CTABr, hexadecyltrimethylammonium chloride; CTACl) at silica-aqueous solution interfaces. The surface force data was analyzed by using DLVO theory. The aggregate of CTABr had smaller surface potential than that of CTACl, and thus, the dissociation degree for CTABr aggregate was lower than that for CTACl. The collapse pressure for CTABr aggregate was smaller than that for CTACl. Although adsorbed aggregates with circular projections were observed for both CTABr and CTACl, the diameter of the circles for CTABr was greater than that for CTACl. Therefore, the aggregates of CTABr on the surface were softer and larger than those of CTACl. It will be considered that the aggregates of CTABr and CTACl were formed with a hemimicelle adsorbed on a flat monolayer and an admicelle on a solid surface, respectively.