The electrical properties of unpassivated wurtzite indium phosphide nanowires were studied by conductive atomic force microscopy. I-V curves of single nanowires with different diameters were obtained. For relatively thin nanowires, surface states caused full depletion of the nanowire volume and low conductivity. With increasing diameter, the conductivity increased significantly due to formation of an undepleted core in the nanowire. A surface state density (∼6•10¹¹ cm^-2eV^-1) was estimated from the radius and doping level of the nanowire. Additionally, experiments were performed to determine whether a piezoelectric current is generated while bending the nanowires. No current was detected.