Preparation of nanocrystalline nickel oxide thin films and their applications for gas sensors were investigated. Nickel oxide thin films were deposited on a glass substrate by a sol-gel process. The microstructure and surface properties of the prepared sample were studied using X-ray diffraction, scanning electron microscope, and transmission electron microscopy. The prepared nickel oxide films had a nanocrystalline NiO phase with the average grain size of 16 nm. Electronic property measurements indicated that the nickel oxide films had p-type conductivity with the activation energy of electrical conductivity around 0.42 eV. Gas sensing characteristics of NiO films were measured for different reducing gases, such as H-2, CH4, NH3 and mixtures of these gases. The gas sensor response as well as response and recovery time was analyzed to understand sensing mechanism, selectivity and optimal sensing conditions. The cross sensitivity data suggested that this sensor could be used to detect gas molecules selectively in ambient conditions with high repeatability. (C) 2013 Elsevier B.V. All rights reserved.