The electronic properties of GaN nanowires containing AlN/GaN multiple quantum disks, grown on Si(111) and luminescing at 3.55 eV, have been studied with nanometric resolution and have been correlated with their structural and optical characteristics. Profiles of the electrostatic potential across the quantum disks have been obtained by electron holography. Such profiles suggest an AlN/GaN conduction band offset of ∼1:5 eV and an average internal polarization field in the quantum disks of ∼1:8 MV/cm. A simulated energy band profile based on these results agrees with the luminescence measurements. © 2012 The Japan Society of Applied Physics.