A nanoscale electrical characterization of graphene (Gr) contacts to Al x Ga 1-x N/GaN heterostructures has been carried out using conductive atomic force microscopy. The impact of the AlGaN microstructure on the current transport at Gr/AlGaN interface was evaluated considering two Al 0.25 Ga 0.75 N/GaN heterostructures with very different quality in terms of surface roughness and defectivity, ie a uniform and defect-free sample and a sample with a high density of V-defects, that locally cause a reduction of the AlGaN thickness. Rectifying contacts were found on the bare (Gr-free) AlGaN surfaces of both samples, but with a more inhomogeneous and lower Schottky barrier height (Φ B≈ 0.6 eV) in the presence of V-defects with respect to the case of the uniform AlGaN (Φ B≈ 0.9 eV). Very different electrical behaviour was observed for Gr on the two AlGaN samples, ie a low barrier height Schottky contact (Φ B≈ 0.4 …