193nm Al2O3/MgF2 HR mirrors were designed and deposited by electron-beam evaporation, and then annealed in air at different temperatures from 250℃ to 400℃. The center wavelength shifted to shorter wavelength after annealing. And the reflectance of HR region increased but the transmittance changed negligibly little with the increasing of the annealing temperature. The total integrated scattering was measured to distinguish the scattering loss and the absorption loss. The results indicated that the total optical loss in the HR region was absorption dominant rather than scattering dominant. To know clearly the loss-reduction mechanism of annealing, the corresponding single-layers of Al2O3 were deposited and annealed in the same way. The change of optical constants was analyzed to explain the reduction of the total optical loss. And the reflectance of the HR mirrors reached was higher than 98%.