The effect of high-temperature annealing on the structure and properties of single crystal β-phase gallium oxide is reported in this work. The investigated sample obtained by cleaving from a bulk β-Ga2O3 ingot grown by the edge-defined film-fed growth method. Some of the samples were annealed in an oxygen-containing atmosphere at temperatures up to T = 1673 K. The temperature dependences of internal friction and dynamic modulus of elasticity were obtained by the composite oscillator method at a frequency of 100 kHz. Optical absorption spectra were investigated in the wavelength range from 200 nm to 2 μm. It was found that annealing and redistribution of gallium vacancies in beta-phase gallium oxide crystals is accompanied by simultaneous changes in the internal friction in the temperature region around 290 K and in the optical spectrum in the infrared region.