ZnO thin films were epitaxially grown on alpha -Al2O3 (0001) single-crystal substrates by rf magnetron sputtering. The films were grown at substrate temperatures of 550-600 degreesC for 1 h at a rf power of 60-120 W and Ar/O-2 ratios of 1-4. The crystalline structure of the ZnO films was analyzed by four-circle x-ray diffraction (XRD) and Rutherford backscattering (RBS)/channeling. For the ZnO films deposited at 550 degreesC, the full width at half maximum (FWHM) of the XRD theta -rocking curve of the ZnO (0002) plane was found to be increased from 0.16 degrees to 0.3 degrees as the rf power was increased from 80 to 120 W. The in-plane epitaxial relationship of the ZnO film on alpha -Al2O3 (0001) substrates was found to be ZnO [10 (1) over bar0]parallel to alpha -Al2O3[11 (2) over bar0], indicating a 300 rotation of the ZnO unit cell with respect to the alpha -Al2O3 (0001) substrate. For the specimen grown at 600 degreesC, the FWHM of the XRD theta -rocking curve was 0.13 degrees, In RBS/channeling studies, the films, which were deposited at 600 degreesC and 120 W, showed good crystallinity, with a channeling yield minimum (chi (min)) of only 3.5%. whereas chi (min) for the films deposited at 550 degreesC was as high as 50%-60%, indicating poor crystalline quality. In the case of photoluminescence (PL) measurements, sharp near-band-edge emission was observed at room temperature. The FWHM of the PL peal; decreased from 133 to 89 meV at a growth temperature 550 degreesC by increasing the rf power. For the films deposited at 600 degreesC, a FWHM of the PL peak of 75-90 meV was observed, which is the lowest value reported to date. From the results of both XRD and PL measurement, it was found that the crystallinity of the films grown at 550 degreesC improved, but its optical property degraded. With increasing rf power, the films show a deep-level emission in the presence of higher mixtures of Ar:O-2 because a considerable amount of activated oxygen was supplied in the ZnO films with an increase of rf power. From transmission electron microscopy and atomic force microscope analyses, the grain size and defects were found to affect the PL properties. The relationship between optical propel ties and crystal quality is discussed in terms of crystalline structure and grain size.