Chemical force microscopy (CFM) was used to measure the adhesion force between NH2-terminated probe tip and gold substrate modified with self-assembled monolayers (SAMs) of 4-aminothiophenol (4-ATP). The curve of adhesion force as a function of solution pH value (force titration curve) was obtained. The pK(1/2) of surface NH2 group of CATP SAMs obtained from the force titration curve is 5.3, comparable with pK(1/2) = 4.9 determined by the conventional contact angle titration.