The Ag+ were reduced to elemental silver by free-electron from microwave radiation [Ag(NH3)2]OH complex aqueous solution without any reducing agent, and the nano-silver sol was formed under the protection of PVP. UV-Vis absorption spectra show that the absorbance of the colloidal silver nanoparticles increases with the radiation time, but the maximal absorption wavelength does not change. The absorbance keeps stable when the microwave radiation time is up to 5 min at the mass ratio of Ag+/PVP=1:8. TEM observation and image analysis results show that the morphology of nano-silver particles are spherical and monodisperse, with an average diameter of 1.36 nm and the standard deviation of 0.54 nm.