Structure of the ZnO thin films prepared by reactive rf magnetron sputtering is studied concerning sputtering gas composition by using X-ray diffraction (XRD). Ar-O-2 and Ar-He-O-2 mixtures are used as sputtering gases. XRD analysis reveals that Ar affects to the crystal orientation and microstructure of the ZnO films. The addition of He is very effective to activate O-2 molecules and a single phase ZnO film is formed at extremely low O-2 concentration of 5% in the sputtering gas. The sputtering gas composition is an important experimental condition to determine the structural properties of the ZnO thin films. Plasma states monitored by an optical emission spectroscopic system are related to the change in the structural properties of the ZnO films.