Aqueous dispersion of highly stable silver nanoparticles were synthesized using gamma irradiation with Monascus purpureus supernatant molecules as reducing and stabilizing agent. The formation of Nano sized silver was confirmed by its characteristic surface Plasmon absorption peak around 420nm in UV-VIS spectra. The size of the silver nanoparticles can be turned by controlling the radiation dose. Dynamic light scattering (DLS) measurement and Transmission electron microscope(TEM) of the synthesized nanoparticles indicated that the size depend on dose. The irradiation was carried out at doses from 1 to 25kGy. XRD analysis of the silver confirmed the formation of metallic silver. Fourier transform infrared (FTIR) spectroscopic data indicate abounding of Ag nanoparticles with Monascus purpureus supernatant molecules. The obtained Ag nanoparticle dispersion was stable for over 3 months at room temperature.