The preparation of stable, uniform copper nanoparticles by the formation of a complex Cu-soybeans extract and the reduction by sodium borohydride are reported in the present paper. It is a simple process for obtaining copper nanoparticles. The samples have been characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM), which reveal the nature of the nanoparticles. These studies reveal that the particles are mostly spherical in shape and have an average size of 20nm. The UV–vis spectra show that an absorption band, occurring because of Surface Plasmon Resonance, exists at 545nm; the size distribution is measured with DLS.