Photoluminescence from GaN nanopowder: The size effect associated with the surface-to-volume ratio GaN particles of different sizes were separated by a sedimentation technique. We find that a correlation exists between the cathodoluminescence ͑CL͒ spectra of the particles and their average sizes. These results imply that the particles contain a substantial fraction of GaN nanocrystallites ͑NCs͒ whose emission is shifted by quantum size effects and whose proportions differ with the overall size of the particle. X-ray photoelectron spectroscopy data combined with ion etching show that the NCs comprising these clusters have a GaN core surrounded by a gallium oxide shell. The results indicate that size-separated GaN particles may be useful CL sources of light for exciting phosphors over a range of UV wavelengths without the need for alloys such as AlGaInN.