Abstract <p>Ga₂O₃ polycrystalline thin films were deposited by spray pyrolysis method. The films were post-annealed at 700, 900, and 1100°C for 2 h, then the crystal structure, surface morphology and optical properties of the films were studied. As the post-annealing temperature increases, the average crystallite size of the film increased from approximately 6 to 21 nm, and the FWHM of rocking curve for the (400) plane of the β‑Ga₂O₃ decreased from 1.29° to 0.38°. The increase of post-annealing temperature controls phase formation in the films. When post-annealed at 700°C, the Ga₂O₃ film is not completely transformed into β phase. While post-annealed at temperatures ≥900°C, the Ga₂O₃ in the films is all β-Ga₂O₃. Additionally, the increase in post-annealing temperature induced changes in the micro-strains of Ga₂O₃ films, which resulted in a reduction of the bandgap. These findings highlight the critical role of post-annealing temperature in controlling the structural and optical properties of Ga₂O₃ thin films, making it a key parameter for improving the quality of β-Ga₂O₃ films.</p>