Abstract <p>Ga₂O₃ is widely used in power devices, ultraviolet photodetectors and light emitting diodes. Defects, such as oxygen vacancies, gallium vacancies and gallium vacancy-oxygen vacancy pairs, play an important role in the optical property. Ga₂O₃ thin films are grown by electron beam evaporation and post-growth annealing is performed to improve the structural and optical quality. The effects of different annealing parameters (temperatures, atmospheres, durations and gas flow rates) on the optical properties of Ga₂O₃ are investigated. The structural property is continuously improved with the annealing temperature, while the photoluminescence (PL) intensity is the strongest in Ga₂O₃ annealed at 900°C. No PL is observed in the as-grown and low-temperature (&lt;600°C) annealed Ga₂O₃. The PL intensities are significantly enhanced by annealing, especially in the blue–green region. Annealing under O₂ at above 900°C eliminates the near ultraviolet emission, which is from the self-trapped excitons, while annealing under N₂ or vacuum has no such effect. Among the samples annealed in vacuum, N₂ and O₂, the sample annealed in O₂ shows the best structural and optical properties. For the annealing durations increasing from 0.5 to 1.5 h, the PL intensity first slightly increases and then decreases. The PL intensity of samples annealed in O₂ with different flow rates shows similar trend.</p>