The copper nanoparticles hydrosols have been successfully synthesized by the chemical reduction of copper (II) sulfate with sodium borohydride at room temperature without any protective agent and inert gas protection. The effect of reaction conditions (copper salt and sodium borohydride concentrations, their molar ratio, pH, surfactants, temperature and others) on the formation copper nanoparticles, its yield and stability in aqueous solution were examined. The products were studied by UV-vis spectroscopy, X-ray photoelectron spectroscopy, X-ray powder diffraction, scanning electron and atomic force microscopy. The copper nanoparticles hydrosols were found to contain generally spherical copper nanoparticles (20-40 nm) with metallic core coated by a copper(I) or copper(II) oxide and hydroxide film.