It is important to prepare high-quality, ultrathin dielectric insulators on graphene for the integration of graphene field-effect transistors. Here, we present a novel and facile approach to deposit high-kappa dielectrics (such as Al2O3) by atomic layer deposition (ALD) through water physisorption. Usually it is difficult to grow Al2O3 thin films on pristine graphene directly using H2O as precursor. The pretreatment of graphene with ALD H2O pulses before the deposition can enhance the Al2O3 coverage on pristine graphene, but the film is not continuous yet. It is found that uniform Al2O3 thin films can easily depositon the graphene after H2O dipping pretreatment for similar to 4-5 h. It is ascribed to the fact that the physisorbed water molecules on the surface of graphene as nucleation sites react with the metal precursor of trimethylaluminum (TMA) to form Al2O3 dielectric films. Raman spectra reveal that no defects are produced on graphene surface via this method. These results indicate that water functionalization is a promising pathway to achieve scaled gate dielectrics on graphene without introducing any defects. (C) 2013 Elsevier B.V. All rights reserved.