Y-doped Al2O3 dielectric films have been fabricated by reactive radio frequency co-sputtering method. Grazing angle incidence x-ray diffraction results show that the as-deposited and annealed films are amorphous. High resolution scanning electron microscope and atomic force microscope have been applied to observe the cross-section and the surface morphology of the thin films. The electric C-V and I-V characteristics were measured at high and variable frequency, respectively. It was found that the dielectric constant k of the films increases remarkably (from 8.14 to 11.8) with increasing Y-doping concentration. The Y—O bond is stronger than Al—O due to the obvious difference in electro-negativity between the two bond members, which enhanced the ionic polarization in the thin films leading to an increase of the dielectric constant. It was supposed that the presence of Y ions changed the structure and atomic coordination of Al2O3. The films were very smooth which meet the requirements of the device.