ZnO (zinc oxide) thin films have been prepared by using rf magnetron sputtering on a SiO2/Si substrate. The crystallographic properties and optical properties were investigated. It is shown that the ZnO films exhibit strongc‐axis orientation. The standard deviation of thecaxis was obtained less than 1°. The optical propagation loss was less than 2 dB/cm (TE0mode) in a 0.6‐&mgr;m‐thick film. To achieve the ZnO films with low light propagation loss, it was found that we must take into account not onlyc‐axis orientation of ZnO films but also the density of ZnO films.