beta-Ga2O3, a native defect luminescent material with blue emission, was fabricated by annealing beta-Ga2O3 powders under various O-2 partial pressures or in Ga metal atmosphere. Annealing under low O-2 partial pressure favors the photoluminescence (PL), which indicates that V-O(x) should be one of the luminescence centers in this phosphor. Annealing in Ga metal atmosphere reduced the PL intensity, which indirectly suggests that V-Ga(x) or defect cluster (V-Ga,V-O)(x) might be another luminescent center. Electron paramagnetic resonance (EPR) measurements verified the existence of single ionized oxygen vacancy (V-O(center dot)). An EPR signal with g=4.0468 and four hyperfine lines was observed, which is considered to be related to Ga2+ and the hyperfine interaction parameter is 112 G. Furthermore, the correlations between PL intensity and the densities of V-O(center dot) as well as Ga2+ were studied. At present, although we can not clarify whether Ga2+ exists as Ga-i(center dot center dot) or Ga-Ga('), V-O(x), and Ga2+ may act as donors, whereas V-Ga(x) or (V-Ga,V-O)(x) may act as accepters for the luminescence. (c) 2007 The Electrochemical Society.