The highly pure nano-silver sols were prepared by an electrolysis method using two highly pure silver flakes as electrodes and deionized water as electrolytic solution, and PVP served as stabilizer. The effects of PVP content, electrolytic time and current density on the colloidal silver nanoparticles were researched. The results indicate that as-synthesized particles were spherical about 1~3 nm in size, and monodispersed and its concentration reached to 130 μg/g under the condition of 5.0 wt% PVP with current density about 1~2 mA/cm2 for 150 min. Moreover, the nano-silver sol had such an excellent stability that it had not any change though it was placed in dark at room temperature for 6 months.