Thermal annealing effects on the microstructures and optical properties of Ge-doped SiO2 films fabricated by flame hydrolysis deposition were investigated. Microstructure modifications from rough to smooth were measured by atomic force microscope at different annealing temperatures. The refractive index (n) and extinction coefficient (k) were obtained by variable angle spectroscopic ellipsometry. It is concluded that k decreased and n increased with increasing annealing temperature. The results suggest the improvement of the film quality can be achieved by thermal annealing. (C) 2003 Elsevier B.V. All rights reserved.