In this article we demonstrate the use of an ozone based functionalization scheme of graphene surface for depositing thin, high-k dielectrics using atomic-layer deposition (ALD) technique. It is found that the ozone functionalization at room temperature provides nucleation sites for atomic layer deposition without causing any substantial damage to the electronic properties of graphene devices. The capacitance of the Al2O3 films was extracted using DC and CV measurements and was found to be similar from both types of measurements. A charge carrier mobility of upto ∼5000 cm2/V-s at room temperature was extracted for top-gated graphene devices with ALD Al2O3 deposited using ozone functionalization.