ZnO thin films were deposited onto glass substrates by d.c. reactive magnetron sputtering from a metallic zinc target. QI systematic study has been made on the influence of sputtering pressure in the range from 0.2 Pa to 3 Pa on the film structural and optical properties. At low sputtering pressure (0.2-0.4 Pa), the film was inhomogeneous, non-stoichiometric and had low refractive index and an almost amorphous structure. At high sputtering pressure (0.6-0.8 Pa), the film was homogeneous, stoichiometric and had high refractive index and the crystallinity was improved. As the sputtering pressure was further increased (1-3 Pa), the homogeneity and the refractive index of the film had no clear variation, but the crystallinity of the film went down. As the sputtering pressure was increased from 0.2 Pa to 3 Pa, the transmittance of the film increased and the deposition rate of the film decreased.