Abstarct: Gas sensors are used to detect toxic and flammable gases in energy storage and environmental protection, which is a necessity. Recently, ZnO / rGO nanocomposites have received much attention in various applications, including gas sensors, due to their excellent electrochemical and optical properties. In this study, samples of pure ZnO and ZnO/rGO nanocomposites with weight percentages of 1, 2, 5 and 8% of GO were prepared using hydrothermal method. The structural, optical and surface morphological properties of the samples were studied using different characterization techniques. The results of these studies showed successful synthesis of ZnO/rGO nanocomposites by hydrothermal method. X-ray diffraction showed that the samples had a hexagonal crystal structure. Scanning electron microscopy showed the growth of ZnO nanocrystals on rGO sheets. Also, with increasing the percentage of GO in the samples, the size of the grains and the band gap of the samples decreased. Parameters of the ideal gas sensor such as sensitivity, response time and recovery and selectivity were evaluated by sensor devices fabricated using ZnO/rGO nanocomposites with weight percentages of 2, 5 and 8% of GO in different concentrations of target gas and at a temperature of 50 ̊C. The sensor fabricated using ZnO/rGO nanocomposites with weight percantage of 8% of GO was considered the best sample with a fast response time of 0.4 s and recovery time of 4.6 s and showed a very high sensitivity to ammonia gas. This piece seems to be a good option for an ammonia gas sensor