Water-soluble Au nanoparticles were prepared using the phase transfer method, and their dispersion properties were investigated. Using mercaptocarboxylic acids having various alkyl chain lengths as stabilizing agents, Au nanoparticles stably dispersed in aqueous solutions were obtained. The Au core size of the nanoparticles decreased with increasing alkyl chain length of mercaptocarboxylic acid. Dynamic lightscattering measurements showed that the size of dispersed Au nanoparticles coated with 6-mercaptohexanoic acid remained almost constant with changing solution pH, while Au nanoparticles coated with 15-mercaptopentadecanoic acid increased with decreasing solution pH, and completely precipitated at pH 5. These observations indicate that the dispersion behavior of the Au nanoparticles depends on the alkyl-chain length of the stabilizing agent, and that is sensitive to the solution pH in the case of the particles with longer alkyl chains.