Nb2O5 films with the thickness (d) ranging from 55 to 2900 nm were deposited on BK-7 substrates at room temperature by a low frequency reactive magnetron sputtering system. The structure, morphology and optical properties of the films were investigated by X-ray diffraction, atomic force microscopy and spectrophotometer, respectively. The experimental results indicated that the thickness affects drastically the structure, morphology and optical properties of the film. There exists a critical thickness of the film, d(cri) =2010 nm. The structure of the film remains amorphous as d < d(cri). However, it becomes crystallized as d > d(cri). The root mean square of surface roughness increases with increasing thickness as d > 1080 nm. Widths and depths of the holes on film surface increase monotonously with increasing thickness, and widths of the holes are larger than 1000 nm for the crystalline films. Refractive index increases with increasing thickness as d < d(cri), while it decreases with increasing thickness as d > d(cri). In addition, the extinction coefficient increases with increasing thickness as d > d(cri). (c) 2006 Elsevier B.V. All rights reserved.