Pd-M (M=Ag and Ni) and Pt-M (Ru and Ni) alloy colloids were successfully prepared in aqueous solution by γ-irradiation using poly(vinylpyrrolidone), PVP, as stabilizer. The PVP-stabilized Pd-M and Pt-M nanoparticles were characterized by UV-Vis spectroscopy, Transmission Electron Microscopy (TEM) and Electrophoretic Light Scattering (ELS) analysis. The influence of molecular size of the PVP on the size and size distribution of the alloy nanoparticles was followed. Pd-Ag nanoparticles were formed by employing PVP with different molecular weights. The size of Pd-Ag alloy nanoparticles was determined by TEM photograph and ELS spectra, respectively. From the TEM photographs, the average diameter of Pd-Ag nanoparticles does not show strong dependence on the molecular weight of the PVP. On the other hand, the average diameter of Pd-Ag alloy colloids prepared by PVP with Mw=40,000 was consistently larger than that of Pd-Ag alloy colloids prepared by PVP with Mw=10,000. A plausible scheme is given to explaining this. The size and size distribution of Pt-M (Ru and Ni) alloy colloids are presented.