The effects of rapid thermal annealing on band structure of HfO2 have been investigated using combinational characterization techniques including X-ray diffraction, X-ray photoelectron spectroscopy and ultraviolet-visible spectrophotometer. The results show that the band structure of the thin film is strongly affected by crystalline structure. Different microstructures of HfO2 formed after annealing at a series of temperatures up to 1000 degrees C: amorphous below 450 degrees C, polycrystalline over 450 degrees C and good crystalline texture above 750 degrees C. The structure phase of polycrystalline thin films in theses samples remains basically unchanged. The HfO2 thin film shows a series of band structures corresponding to different microstructure properties. The further study shows that the band-gap energy increases after the polycrystallization of HfO2. Both valence band and conduction band shift downward, with a relatively smaller shift of the conduction band. A trap level forms below the conduction band after the polycrystallization of HfO2 thin films. (c) 2005 Elsevier BY All rights reserved.