In this paper, we present the use of keV and MeV ion beam irradiations and post-irradiation annealing to form Cu nanoparticles in fused silica glass. The sequence of process being: low energy irradiation of fused silica substrate, Cu thin film coating, SHI irradiation and annealing. Fused silica substrates were irradiated using 30 keV argon ions at fluences of 1 Γ 10 17 ion/cm 2 .10 nm Cu films were deposited on the low energy irradiated fused silica. These thin films coated fused silica samples were then irradiated using 120 MeV Ag 9+ ions at fluences ranging from 4 Γ 10 13 to 1 Γ 10 14 ion/cm 2 . The irradiated samples were subjected to isochronal annealing in argon gas flow from 400 K to 1200 K for 30 min. The average sizes of Cu nanoparticles were estimated from optical absorption spectra using Mie's theory. The sizes of the Cu nanoparticles were also confirmed from GAXRD and TEM measurements. Postirradiation annealing results indicate that the formation of Cu 3 nanocluster is favored at annealing temperature of about 700 K. High temperature annealing at 1200 K causes the formation of Cu 4 clusters as well as Cu nanoparticles (larger clusters). After annealing at 1200 K, for the sample 120 MeV Ag 9+ irradiated with high dose (1Γ 10 14 ion/cm 2 ) only Cu nanoparticles are formed. The study suggests that irradiation induced defects provide nucleation sites for the formation of nanoparticles, latent tracks provide fast diffusion channels for Cu and, annealing provides required energy for the diffusion.