We have prepared indium nitride (InN) thin films on ZnO/alpha-Al2O3 by RF reactive magnetron sputtering and studied their optical and electrical properties using such analytical techniques as X-ray diffraction (XRD), van der Pauw technique, atomic force microscopy (AFM) and double-beam spectroscopy. ZnO-buffered alpha-Al2O3 is effective to the epitaxial growth of InN thin films and, furthermore, improve their electrical properties. ZnO buffer layers transmit the information on c-axis orientation from (0001) alpha-Al2O3 substrates to InN thin films, which grow epitaxially along lateral orientation because of less lattice mismatch between InN and ZnO than that between InN and alpha-Al2O3. The maximum Hall mobility is 60 cm(2)/V.s and the minimum carrier concentration is 2 x 10(20) cm(-3). From optical transmission spectra, optical constants of the films are obtained based on Kramers-Kronig analysis. The refractive index n is around 1.8 in the photon energy of 1.0-2.5 eV and the band gap is approx. 1.8 eV.