GaAS/SiO2 nanogranular thin films have been fabricated by radio frequency magnetron co-sputtering technique and postannealing. The existence of GaAs nanocrystals in the SiO2 matrix of the films is supported by the analysis of the X-ray diffraction patterns and the characteristic of the room temperature Raman spectra. The average size of the GaAs nanocrystals was estimated to be 1-3nm using the Scherrer formula. The compositions of GaAs and SiO2 in the films are demonstrated to be stoichiometric by the Rutherford backscattering spectra. Raman red shifts mainly result from the phonon quantum confinement in the GaAs nanocrystals. Room temperature optical absorption spectra exhibit obvious absorption edge blue shifts with respect to the absorption edge of bulk GaAs. The blue shifts calculated by effective mass model are much larger than those of the experiment. This may result from the exponential increase in the effective mass of the exciton in the GaAs nanocrystals with the decrease of the average radius of the nanocrystals. (C) 2002 Elsevier Science B.V All rights reserved.