Structural, electrical and optical properties of indium tin oxide or ITO (In2O3:SnO2) thin films on different substrates are investigated. A 100-nm-thick pre-deposited zinc oxide (ZnO) buffer layer is utilized to simultaneously improve the electrical and optical properties of ITO films. High purity ZnO and ITO layers are deposited with a radio frequency sputtering in argon ambient with plasma powers of 150 W and 300 W, respectively. After deposition, samples are annealed in a high vacuum furnace at 400 ˚C. The effects of ZnO-coated substrates on the crystallinity and morphological properties of ITO films are analyzed by X-ray diffractometer, field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). X-ray diffraction patterns confirm the hexagonal wurtzite type polycrystalline structure of the ZnO films. FESEM and AFM analyses indicate that the surface morphology of the ITO films is affected by the ZnO buffer layer. Results also reveal that the roughness of ITO thin films is decreased in presence of the ZnO buffer layers. It has been found that ZnO incorporation promotes the crystallization of the ITO layer reduces its resistivity without deteriorating the optical transmittance.