US nanoparticles (CdS NPs) were prepared in an aqueous solution with poly(allylamine hydrochloride) (PAH) as a stabilizer. It was found that the size of US NPs was easily controllable with solution pH with good reproducibility. By comparing the pH dependence of US NPs stabilized with poly(acrylic acid) (PAA) having an opposite charge to PAH, the electrostatic interaction amongst the polyelectrolyte was believed to have a great influence on the size determination.