Mg codoping into Eu-doped GaN (GaN: Eu) changed the dominant optical site and increased the photoluminescence (PL) intensity at room temperature (RT). From the ratio of PL integrated intensity at 25 K to that at 300 K, PL efficiency of the GaN: Eu, Mg layer was evaluated to be as high as 77%. On the basis of this experiment, GaN: Eu-based LEDs grown by NH 3 MBE were fabricated. Clear rectification characteristics with a turn-on voltage of 3.2 V were observed and a pure red emission was observed by the naked eye at RT. For the electroluminescence (EL) spectra, two predominant peaks of higher-efficiency optical sites A and C were selectively enhanced and the EL intensity was improved. This result suggests that GaN: Eu was very effective for realizing red-light-emitting devices using the nitride semiconductor.