Au/Pd bimetallic nanoparticles, even with core-shell structure, were synthesized by successive and simultaneous sonochemical irradiation of their metal precursors in ethylene glycol, respectively. In the successive method, Pd clusters or nanoparticles are first obtained by reduction of Pd(NO 3) 2 , followed by adding HAuCl 4 solution. As a result, Au-core/Pd-shell structured particles are formed, instead of Pd-core/Au-shell as usually expected. The successive method is more effective than the simultaneous one in the formation of the core-shell structure. Detailed investigation with optical absorption spectroscopy suggested that the pre-formed Pd atoms or clusters have a reduction effect on Au 3+ ions and the post-formation of Pd-shell can damp the surface plasmon resonance of Au nanoparticles. Theoretical absorption spectra based on Mie-like model for core-shell structured particles yield excellent agreement with the experimental results.