Abstract: The effects of annealing on electron-beam evaporated single Al2O3 layers were investigated. The films were annealed in air for 1.5 hours at different temperatures from 250～400 ℃. The transmittance spectra were measured in the wavelength range of 190~400 nm. Microstructures of the samples were characterized by X-ray diffraction (XRD). Profile and surface roughness measurement instrument was used to determine the rms surface roughness. It was found that the transmission spectra shifted to short wavelength gradually as the annealing temperature increased and the total optical loss decreased. The film structure remained amorphous even after annealing at 400 ℃ temperature. And the samples annealed at higher temperature had the higher rms surface roughness. The decreasing optical loss with an