We investigated the correlation between structural and photoelectrochemical properties of GaN porous nanostructures formed by photo-assisted electrochemical etching. The porous nanostructures were formed during light irradiation of the top-surface of homo-epitaxial layers grown on freestanding GaN substrates. The pore depth, wall thickness, and surface morphology of porous nanostructures were strongly influenced by the way holes generated by the light irradiation were supplied. Such structural features influenced the optical properties of GaN porous nanostructures. The photoluminescence peaks measured on GaN porous nanostructures were shifted to higher energies because of the quantum confinement in the thin GaN walls between pores. Formation of porous nanostructure decreased the photoreflectance of the GaN surface, and the smallest reflectance was obtained from the porous sample having large pores on its surface after the ultrathin layer with small pores had been removed by surface-etching. The photoelectrochemical response measured on GaN porous nanostructures in a NaCl electrolyte were drastically enhanced by the unique features of those structures, such as low photoreflectance and large surface area. The largest photocurrents were obtained from the sample from which H 3 PO 4 treatment had removed the ultrathin layer without thinning the pore walls. Photoelectrochemical systems based on semiconductor photoelectrodes have recently attracted much attention due to their potential use in the next generation of green technologies such as water splitting for fuel cells, artificial photosynthesis, and so on.1-5 Among the photoelectrode materials, GaN is one of the most attractive because of its chemical stability and its potential to achieve direct photoelectrolysis by solar power without the consumption of electric power. [6][7][8] In addition, the bandgap energy of GaN-based materials can be varied from about 0.65 to 6.0 eV by alloying them with InN and AlN, which enables us to design various functional photoelectrodes not only for spectral matching of solar light but also for the electrochemical reduction of CO 2 to carbohydrate. 9 One of the common approaches to improving conversion efficiency is to form nanostructures on the photoelectrode surface in order to increase its surface area. Most reported GaN nanostructures have been made using selective-area growth 10,11 or a dry etching process such as reactive ion etching. 12,13 There are, however, severe limitations on increasing the density of nanostructures because most approaches use lithography for defining the size and position of the nanostructures. And when a dry etching process is involved, the etching damage induced by ion bombardment is not negligible [14][15][16] and could significantly degrade the photoelectrochemical efficiency.One alternative approach is an electrochemical-fabrication process, which can form various semiconductor nanostructures in a self-assembled fashion. The most well-known application of an electrochemical process is the formation...