Size-controlled gold nanoparticles, ranging from 2 to 15 nm in diameter, were first prepared from gold substrates in 0.1 N HCl aqueous solutions without the addition of any stabilizers by sonoelectrochemical methods. First, an An substrate was cycled in a deoxygenated aqueous solution containing 0.1 N HCl from -0.28 to +1.22 V vs Ag/AgCl at 500 mV/s with 100 scans. The durations at the cathodic and anodic vertexes are 10 and 5 s, respectively. After this process, Au- and Cl-containing complexes were left in the solution. The Au working electrode was then immediately replaced by a Pt electrode, and cathodic overpotentials of 0.2, 0.4, and 0.6 V from the open circuit potential (OCP) of ca. 0.82 V vs Ag/AgCl were applied under sonification to synthesize An nanoparticles. The prepared nanoparticles increase in size from 2 to 15 nm with the increase of the cathodic overpotential applied from 0.2 to 0.6 V. Furthermore, the ratio of An nanoparticles to Au-containing nanocomplexes in solutions can be controlled by adjusting the sonoelectrochemical reduction time.