Abstarct: Nowaday, due to the speared of hazardous gases, control and immunization of the environment from dangerous has a special significance. mextal oxide gas sensors are used for these proposes. Some of advantages of these are high sensitivity, simple design, and relatively low weight and low cost. Main characteristics of these sensors such as their sensitivity and selectivity are affected directly by the choice of working material. In this thesis, two of the most widely used mextal oxides were chosen and investigated. Tin oxide (SnO2) nanowires were synthesized by chemical vapor deposition (CVD) method. Also, iron oxide (Fe2O3) nanostructures were grown using physical vapor deposition (PVD). Prepared samples were characterized using scanning electron microscopy (SEM) and x-ray diffraction (XRD) analysis. SEM images were showed nanostructure for tin oxide samples and rhombohedral structure at the preferred (113) lattice direction. The sensing properties of samples were measured toward methane, ethanol, and LPG gas species. Working temperature for methane, ethanol, and LPG gases were determined to be 250, 250, and 270 ˚C. Additionally, the effect of gas concentration on the response of sensors were examined for all of these different gases at working temperature. In order to increasing the sensitivity and improving the selectivity of samples, Nickel (Ni) mextal was used as a catalyst. For this purpose a thin laxyer of NiO was depostited on top of SnO2, Fe2O3 samples using spray pyrolysis technique. This process increased the sensitivity of SnO2, Fe2O3 from 57% for bare laxyers up to 90% for laxyers covered with NiO thin laxyer. Also, the selectivity of SnO2, Fe2O3 samples were increased toward ethanol. At the end of this context some suggestions for future works have been proposed.