Binary Au-Pd alloy nanoparticles were prepared by ultrasound irradiation of solutions containing Au(3+), Pd(2+) ions and surfactant, and their characteristics were examined. It was found that nanoparticles with a core-shell structure of Au-core and Pd-shell exhibited superior hydrogenation catalytic activity to Au-Pd alloys or pure Pd nanoparticles. In order to examine the electronic states of nanoparticles, coincidence Doppler broadening (CDB) measurements have been performed for the nanoparticles and bulk materials of Au-Pd alloys. The electronic state of Au-Pd nanoparticles prepared by sonochemical reduction was shown to be slightly different from that of pure Pd black of a commercially available catalysis.