The in-plane orientation of epitaxial ZnO thin film on Al2O3(0 0 0 1) was determined by azimuthal scan of X-ray diffraction. Comprehensive structural characterizations, including the lattice strain in perpendicular direction, the defect density, were obtained from high resolution X-ray diffraction. It's found that a 30° rotation in ZnO against Al2O3, resulting in ZnO〈1 1 2 0〉//Al2O3〈1 0 1 0〉, can efficiently reduce the strain and defects in ZnO layer. Consequently, the optical property is significantly improved.