Abstarct: In this thesis, nickel oxide nanostructures with different annealing temperatures were prepared by hydrothermal method and then their structural, optical properties as well as their sensing efficiency in the presence of ethanol and acetone gases were investigated. In order to investigate the structural properties of the samples, X-ray diffraction (XRD) measurements and Field Emission Scanning Electron Microscope (FESEM) were used. The XRD spectra of the samples confirmed the formation of the cubic structure of nickel oxide. The results obtained from these spectra showed that the crystallite sizes increase with increasing the annealing temperature. The value of the crystallite sizes obtained from the Debay Scherer relation are in the range of 36-45 nm. Also, FESEM images of the samples demonstrated the accumulation of nanoparticles and formation of rod-shape particles due to increase in annealing temperature. The study of the optical properties of the samples using an UV-VIS spectroscope showed that the band gap of the samples decrease with increasing the annealing temperature. The results of sensor performance of the samples revealed that the samples annealed at 500 ° C and 700 ° C at 250 ° C operating temperatur showed better sensitivity in the presence of ethanol (28.20%) and acetone (24.61%), respectively.