Abstract: Novel direct band gap crystalline nanodiamonds and light emitting devices utilizing the direct band gap crystalline nanodiamonds are disclosed. With providing the detailed information on the electronic states and the electron band structure of several crystalline nanodiamonds, preferred device structures including an electroluminescence-based solid-state light source and a cathode-luminescence-based micro light source are shown. These devices emit light in the region of ultraviolet wavelength, which can be designed from 180 nm to 230 nm by a choice of nanodiamonds, with referring to the information provided in the present invention. The related applications of these UV light emitting devices include a light source for sterilization, a light source for decomposing harmful substances, a light source for spectroscopy, a light source for exciting a phosphor to emit a white light, a micro light source for micro optoelectronic devices, and a light source for writing a super high density recording media.