Gallium oxide (Ga2O3) thin films were deposited on silicon (100) and sapphire (001) substrates using the plasma-enhanced atomic layer deposition (PEALD) technique with an alternating supply of reactant source, [(CH3)2GaNH2]3, and oxygen plasma. The thin films were annealed at different temperatures (500, 700, and 900 °C, respectively) in a rapid thermal annealing system for 1 min. It was found that Ga2O3 thin films deposited by PEALD showed excellent step coverage characteristics. X-ray diffraction measurements showed that the as-deposited thin film was amorphous. However, the thin films annealed at temperatures higher than 700 °C showed a (400) orientation of the monoclinic structure. An atomic force microscope was used to investigate the surface morphologies of the thin films. The thin films showed very smooth surfaces; the roughness of the as-deposited thin film was about 4 Å. With increasing annealing temperature, the thin film became rougher compared with that annealed at lower temperatures. A double-beam spectrophotometer was used to measure the transmittances of the thin films on the sapphire substrates. The thin films showed a very high transmittance (nearly 100%). The band-gap energies of the thin films were determined by a linear fit of the transmittance spectra and were calculated to be between 5.0 and 5.24 eV. The electrical properties of thin films of Pt/film/Si structure were also investigated. It was found that, with increasing annealing temperature, th- e insulating characteristics of the Ga2O3 thin films were significantly improved. Spectroscopic ellipsometry was used to derive the refractive indices and the thicknesses of the thin films. The refractive indices of the thin films showed normal dispersion behavior. The refractive indices of the thin films annealed at low temperatures were smaller than those annealed at high temperatures.