Ultrathin high-k dielectric ErAlO films were deposited on Si (100) substrates by using radio-frequency magnetron sputtering.The very fiat surface of the annealed film with arms roughness less than 0.25 nm was observed by using an atomic force microscope. The film shows good thermal stability when annealing at 900℃ for 30s in the O2 ambient. The effective dielectric constant of the film is around 15.2,and a low leakage current of 8.4 × 10-5 A/cm2 at an electric field of 1 MV/cm was achieved for the film with the equivalent oxide thickness of 2.0nm after annealing.