Abstarct: Nowaday, various methods are used to identify gases. A gas sensor is a type of converter that detects gas molecules and produces signals in proportion to the concentration of the gas. Among the types of gas sensors, mextal oxide semiconductor sensors are the most promising gas sensors that have been used to detect gases even at low concentrations. In this thesis, Zinc Oxide (ZnO) nanostructures have been studied as a gas sensor to detect ethanol vapor. For this purpose, ZnO nanorods and ZnO thin films were selected. A two-step method was used to synthesize ZnO nanorods. In the first step, a thin laxyer of ZnO was coated by spin coating method as a seed laxyer on a glass substrate. Concentration of the deposition solution of seed laxyer was considered as a variable parameter. Then, ZnO nanorods was grown on the seed laxyer using a sol-gel method. Moreover Pulsed laser deposition (PLD) and Dip coating methods were used to synthesize ZnO thin films. In the dip coating method, a deposition solution with different concentrations was used to prepare ZnO thin film samples. The structure and morphology of all samples was studied with X-ray diffraction and scanning electron microscope. In addition, the optical properties of the samples were determined using an Ultraviolet–visible (Uv-Vis) spectroscopy device. A hand-made setup was used to evaluate the gas sensor properties of the samples by measuring electrical resistance of the samples in air and exposed to ethanol vapor. Seven different types of ZnO nanostructures were tested as sensors. Gas sensing results for ZnO nanorods show that these samples have a higher potential for use as a ethanol vapor sensor compared to ZnO thin film samples. The highest response to 100 ppm of ethanol for thin film samples was ~ %27 at 220 °C, which is about 10% lower than the ZnO nanorods sensor response at 220 °C. In addition, the response time in the nanoroads baxsed sensors are smaller than the sensors baxsed on thin film samples, which is a great advantage. For example, the smallest response time for sensors baxsed on ZnO nanorods is measured at about 132 seconds (S-1,100 ppm concentration), while the lowest response time for thin film sensors is about 237 seconds for the same amount of ethanol.