The sub-band-gap photocurrent of an individual defective GaN nanowire was measured using conductive atomic force microscopy (C-AFM) with the conductive tip under illumination. The intrinsic defects introduced additional states in the band-gap and assisted the inelastic tunneling process, resulting in negative differential resistance (NDR). The sample exhibited a photocurrent response at photon energies below the band gap energy of GaN due to the high density of defects in the nanowire. The cathodoluminescence spectrum of the GaN nanowire confirms that the defects give rise to sub-band-gap emissions. The mechanisms of the sub-band-gap photocurrent are discussed. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3586793] All rights reserved.