It is observed for the first time that the interference peaks of transmission spectra of TiO2 film shift to the shorter wavelength after UV illumination. The atomic force microscopy (AFM) was used to monitor the dynamic change of the film surface with UV illumination time, which shows that a similar to 30 nm water layer can be formed on the surface after 90 min UV illumination. The XPS analysis revealed that the water layer is composed of physically absorbed water molecule because it can be wiped off by ultra high vacuum (UHV). At last, based on those analyses, a model was established that uses characteristic matrix and admittance to simulate the effect of UV illumination on the film transmission spectra, and it is found that the film refractive index slightly decreases with UV illumination, which may be due to the absorbed water and other surface structure changes induced by UV illumination. (C) 2008 Elsevier B. V. All rights reserved.