A newly modified sonochemical approach was demonstrated to synthesize stable Au suspensions. The formation of Au nanoparticles with diameters of similar to20 nm was accomplished through the ultrasonic irradiation of aqueous HAuCl4 solutions containing trisodium citrate at 4 degreesC. The efficient generation in gold particles only occurred when the sufficient citrate concentrations (1.9, 2.9, and 3.9 mM) were reached. At low citrate concentrations ranging from 0.49 to 1.5 mM, large aggregates of coalesced Au particles were observed as sonication time increased. It is proposed that citrate could transform as the reducing radicals, 1,2,3-tricarboxy-2-propyl radicals, for the AuCl4 reduction and might also act as the stabilizer during particles formation.