Niobium. oxide (Nb2O5) films with thicknesses ranging from 200 to 1600 nm were deposited on fused silica at room temperature by low frequency reactive magnetron sputtering system. In order to study the optical losses resulting from the microstructures, the films with 500 nm thickness were annealed at temperatures between 600 and 1100 degrees C, and films with thicknesses from 200 to 1600 nm were annealed at 800 degrees C. Scanning electron microscopy and atomic force microscopy images show that the root mean square of surface roughness, the grain size, voids, microcracks, and grain boundaries increase with increasing both the annealing temperature and the thickness. Correspondingly, the optical transmittance and reflectance decrease, and the optical loss increases. The mechanisms of the optical losses are discussed. The results suggest that defects in the volume and the surface roughness should be the major source for the optical losses of the annealed films by causing pronounced scattering. For samples with a determined thickness, there is a critical annealing temperature, above which the surface scattering contributes to the major optical losses. In the experimental scope, for the films annealed at temperatures below 900 degrees C, the major optical losses resulted from volume scattering. However, surface roughness was the. major source for the optical losses when the 500-mn films were annealed at temperatures above 900 degrees C. (c) 2005 Elsevier B.V. All rights reserved.