In recent years, interest has grown in the development of an electronic "nose," capable of detecting mixed gases and even odors. Gas Sensors which are chemical sensors are used for this purpose. Semiconductor metal oxide (SMO) gas sensors have emerged as efficient gas detecting tool. Depending on the type of charge carrier involved in the sensing processes, these devices may be divided into three different groups as (a) electronic conductance sensors (the charge carriers are electrons or holes), (b) ion conductance sensors, (the charge carriers are exclusively ions) and (c) mixed conductance sensors (the charge carriers are both electrons and ions). Nanostructured materials present new opportunities for enhancing the properties and performance of gas sensors because of the much higher surface-to-volume ratio in nanomaterials compared to coarse micro grained materials. Nano structured films with high porosity offer excellent opportunities for gas detection. For gas sensing application, the nano structured thin film topology is very important character; porous topology can build permanent pathways for the test gases. Therefore, porous structure of the gas sensing thin film is more desirable.