Colloidal Au nanoparticles were assembled on a gold substrate via a p-aminothiophenol coupling layer. The surface coverage of Au nanoparticles was found to be closely related to the pH value of gold colloids. In alkaline solution, where the amino group was not protonated, Au nanoparticles could not be adsorbed on the substrate, while, under acidic conditions, protonation makes the amino group positively charged and the adsorption becomes easier. The interaction between Au colloids and the substrate is believed to be electrostatic.