Low Energy Cluster Beam Deposition (LECBD) has been used to synthesize cermets with controlled size nanoparticle. In the Bi-SiOxsystem, this technique allows the formation of Bi nanograins uniformaly distributed in the SiOxinsulating matrix. The size of the Bi nanograins (mean diameter about 4 nm) may be responsible of a surprising optical property of the LECBD Bi-SiOxcermets. The difference between theoretical and experimental optical behavior of LECBD cermets is assumed to be a clue of size effects inducing a metal-semiconductor transition.