Abstract: The core-shell Ag-Ga/polymethyl methacrylate nanoparticles were prepared by the emulsion polymerization method under ultrasonic irradiation in-situ without any initiators or metal reducer. HREM, EDS and XRD experiments were performed to characterize the nanoparticles. The results indicate that the nanocomposite particles possess a core-shell structure with diameters of 80~200nm, as well as excellent monodispersity. The phenomenon that the polymer forms the shell via layer-by-layer self-assembly is found. XRD proves the existence of Ag_0.72Ga_0.28 and the probability of new Ag-Ga alloy because of two unknown diffraction peaks.