High-quality thin insulating films on graphene (Gr) are essential for field-effect transistors (FETs) and other electronics applications of this material. Atomic layer deposition (ALD) is the method of choice to deposit high-κ dielectrics with excellent thickness uniformity and conformal coverage. However, to start the growth on the sp 2 Gr surface, a chemical prefunctionalization or the physical deposition of a seed layer are required, which can effect, to some extent, the electrical properties of Gr. In this paper, we report a detailed morphological, structural, and electrical investigation of Al 2 O 3 thin films grown by a two-steps ALD process on a large area Gr membrane residing on an Al 2 O 3 –Si substrate. This process consists of the H 2 O-activated deposition of a Al 2 O 3 seed layer a few nanometers in thickness, performed in situ at 100 °C, followed by ALD thermal growth of Al 2 O 3 at 250 °C. The optimization of the low-temperature seed layer allowed us to obtain a uniform, conformal, and pinhole-free Al 2 O 3 film on Gr by the second ALD step. Nanoscale-resolution mapping of the current through the dielectric by conductive atomic force microscopy (CAFM) demonstrated an excellent laterally uniformity of the film. Raman spectroscopy measurements indicated that the ALD process does not introduce defects in Gr, whereas it produces a partial compensation of Gr unintentional p-type doping, as confirmed by the increase of Gr sheet resistance (from ∼300 Ω/sq in pristine Gr to ∼1100 Ω/sq after Al 2 O 3 deposition). Analysis of the transfer characteristics of Gr field-effect transistors (GFETs) allowed us to evaluate the relative dielectric permittivity (ε = 7.45) and the breakdown electric field ( E BD = 7.4 MV/cm) of the Al 2 O 3 film as well as the transconductance and the holes field-effect mobility (∼1200 cm 2 V –1 s –1 ). A special focus has been given to the electrical characterization of the Al 2 O 3 –Gr interface by the analysis of high frequency capacitance–voltage measurements, which allowed us to elucidate the charge trapping and detrapping phenomena due to near-interface and interface oxide traps.