Gallium oxide (Ga2O3) thin films were prepared on c-plane sapphire substrates by plasma enhanced atomic layer deposition (PEALD). The effects of annealing atmosphere (v(N2)∶v(O2)=1∶1 (volume ratio), Air and N2) and annealing time on the crystal structure, surface morphology and optical properties of Ga2O3 films were studied. The research results indicate that gallium oxide is in metastable state before annealing, and the crystal structure changes significantly after annealing in different annealing atmospheres, and the increase in the proportion of N2 in annealing atmosphere is beneficial to the recrystallization of Ga2O3. Furthermore, the effect of annealing time was further studied under N2 atmosphere, and the results show that the structure of the thin film changes from metastable state to β-Ga2O3 with good single orientation after annealing for 30 min in N2 atmosphere. And the surface morphology analysis show that the surface morphology begins to stabilize after annealing for 30 min, and the surface grain density no longer increases. In addition, the average transmittance of the sample in the range of 400 nm to 800 nm is almost 100%, and the light absorption edge is steep. Annealing in N2 atmosphere is more conducive to the migration of atoms on the surface of the film for Ga2O3 deposited in an oxygen-rich environment, and Ga2O3 recrystallization is preferred.