A simple chemical reduction method is used to prepare colloidal bimetallic Cu-Ag core-shell (Cug) nanoparticles. Polyvinyl pyrrolidone (PVP) was used as capping agent, and ascorbic acid (C6H8O6) and sodium borohydride (NaBH4) were used as reducing agents. The obtained Cug nanoparticles were characterized by powder x-ray diffraction (XRD), transmission electron microscopy (TEM) and UV-vis spectrophotometry. The influence of [Ag]/[Cu] molar ratios on the fonnation of Ag coatings on the Cu particles was investigated. From the TEM results the authors found that the ratio [Ag+]/[Cu2+] =0.2 is the best for the stability of Cug nanoparticles with an average size of 22 nm. It is also found out that adding ammonium hydroxide (NH4OH) makes the obtained Cug nanoparticles more stable over time when pure deionized water is used as solvent.