HfSixOy thin films are prepared by using radio-frequency (RF) magnetron sputtering on p-type Si(100) and then annealed in N2 atmosphere. Composition and structure of the films are determined by means of XPS and XRD, respectively. The optical properties in relation to postdeposition annealing temperatures are investigated by spectroscopic ellipsometry(SE). The XRD analysis shows that the films are amorphous even after annealed at 700 ℃ but crystallization after annealed at 900 ℃. Based on a parameterized Tauc-Lorentz dispersion model, the optical constants of the as-deposited and annealed films related to the annealing temperature are systematically extracted. With the increase of the annealing temperature, the refractive index n of the HfSixOy films increases whereas the extinction coefficient k of the HfSixOy films decreases. The extracted direct band gaps are 5.62, 5.65, 5.68 and 5.98 eV for the as-deposited films and the films annealed at 500 ℃, 700 ℃ and 900 ℃, respectively.