The work presents the results of the experiments on annealing in air of bulk crystals of gallium oxide grown in Ar+O2 and CO2 atmospheres at a temperature of 1400 °C. The annealing time was 5 hours; the time to reach the temperature was 3.5 hours; the cooling time was 20 hours. Annealed samples show increasing of transmission in infrared area of electromagnetic spectrum and decreasing of width of X-ray rocking curve which means the reduction of the number of defects in crystals. Full width at half maximum of rocking curve for annealed samples was almost the same for both atmospheres: FWHMa = 84 arcsec for sample grown in Ar+O2 atmosphere and FWHMa = 80 arcsec for sample grown in CO2, which means that after annealing, the quality of the samples became comparable, despite the initial difference.