The synthesis of gold nanoparticles with controllable particle sizes and densities on platinum substrates was first developed by sonoelectrochemical methods in this study. First, an Au substrate was cycled in a deoxygenated aqueous solution containing 0.1M HCl from −0.28 to +1.22V vs. Ag/AgCl at 500mV/s with different scans. The durations at the cathodic and anodic vertexes are 10 and 5s, respectively. After that Au-containing complexes were found existing in this aqueous solution. Subsequently, the Au working electrode was immediately replaced by a Pt electrode and different cathodic overpotentials and reduction time were employed under controlled sonication to synthesize Au nanoparticles on the Pt electrode. Then pyrrole (Py) monomers were encouragingly found to be autopolymerized to form polypyrrole (PPy)/Au nanocomposites on the Pt substrate due to the electrochemical activity of some unreduced species of the positively charged Au-containing complexes, which were incorporated into the deposited Au nanoparticles during the sonoelectrochemical reduction in a specific condition.