Ga2+xO3-x thin films grown on sapphire substrates by metal-organic chemical vapor deposition under different conditions (temperature pressure) are studied by rutherford backscattering spectrometry/channeling. The structural information and crystalline quality are further investigated by high resolution X-ray diffraction (HR-XRD). The results suggest that at the same growth-temperature the crystalline quality is improved with pressure decreasing, while χmin reaches a minimum 14.5% when the pressure decreases to 15 Torr (1 Torr=133.322 Pa). Then if the pressure is kept at 15 Torr, all films present similar crystalline qualities, which hints that the temperature is not a chief factor. Moreover, films prepared under the same condition are annealed at different temperatures: 700, 800 and 900 ℃. At first the crystalline quality is improved by increasing the annealing temperature and reaches a best χmin of 11.1%. Nevertheless, as the annealing temperature is further increased, the samples become decomposed. XRD spectra of annealed samples each reveal a strong peak of Ga2O3 (402), indicating that the epitaxial layer has a preferred orientation (402).