A simple method was established based on the envelope curves of the optical transmission spectrum over the wavelength range of 200 to 2500 nm at normal inc idence, which was used for the accurate determination of the optical constants o f the anodic aluminum oxide(AAO) films. The results showed that the AAO films exhibited the optical features of a semiconductor with direct band gap of about 45 eV, and the optical constants of the AAO films depended strongly on the ano dic oxidation voltage, an important technologic parameter for preparation of AAO films. With the increase of the anodic oxidation voltage, the optical constants including the refractive index, thickness and optical band gap of AAO films inc reased, and the extinction coefficient decreased. Meanwhile, the fact that the c alculated values of the thickness of AAO films were in satisfactory agreement wi th the values of measurement illuminated that the results were well self_consist ent with the experiment.