The post-annealing process is an effective method to improve the properties of gallium oxide (Ga₂O₃) single crystal films. However, the studies on post-annealing of Ga₂O₃ films have been largely limited to the annealing temperature, with few reports of the annealing time. Here, we show the effect of annealing time on the Er-doped β-Ga₂O₃ (−201) epitaxial films deposited by pulsed laser deposition. The Ga₂O₃ film with a bandgap of ~ 4.98 eV and annealed at 900 °C for 15 min, exhibits the best crystal quality and optical property in comparison with the films before and after the annealing for 7 and 30 min in air. The epitaxial relationship between the film and substrate is Ga₂O₃ [010] ∥ GaN [−12−10] with Ga₂O₃ (−201) ∥ GaN (0001). Moreover, the resistivity of the 15 min-annealed Ga₂O₃ film, which is about 180 Ω·cm, still maintains a low level.