ZnO thin films were deposited onto Al/Si (Al film on Si) substrates by r.f. magnetron sputtering of a ZnO target, The effects of the oxygen content on the structural characteristics and the electrical resistivity were studied as a function of the ratio of the sputtering gas mixture (Ar-to-O-2 ratio). As the Ar-to-O-2 ratio decreased, the preferred orientation also decreased owing to the resputtering effect of the neutral oxygen. The resputtering of the ZnO film enhanced the grain growth. The width of the interface layer between the ZnO film and the Al/Si substrate increased with increasing oxygen content owing to resputtering. The ZnO film composition as a function of thickness and gas mixture was nearly exactly stoichiometric. As the Ar-to-O-2 ratio decreased, the resistivity increased owing to the decrease in preferred orientation.