In this letter, copper nanoclusters formation was observed in Cu-doped silicate glasses after UV-light irradiation followed by thermal annealing. After heating at 460 degrees C for 45 min, the Cu nanoclusters formed only in the UV-light irradiated glass, but not appeared in the un-irradiated sample. The critical temperature around at 460 degrees C was found in the irradiated sample to form Cu nanoclusters. Thermal annealing at the temperature above this critical point would form Cu nanoclusters both in the irradiated and un-irradiated glasses. Compared to the un-irradiated sample, the UV-light irradiated glass usually had more free electrons, and then more Cu(+) could trap electrons to form atoms. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.