Abstarct: The life of human being has been grown over the past decades due to the industrial revolution. The negative aspect of this industrialization is the production of gas species that pollutes the environment and threats public health. Gas sensors are used to measure the levels of these contaminants which have been fabricated from different sensing materials for variety of different gases. In this research, polyaniline was first synthesized by oxidative chemical polymerization route and then polyaniline/zinc oxide composite was synthesized with the same route in the presence of zinc oxide powder. After purification, the polyaniline and polyaniline/ZnO composite was disolved in water and deposited on top of pre gold coated, as electrodes, glass substrates using spin coating system for fabricating the gas sensor. Structural, optical and morphological properties of synthesized samples was carried out using X-ray diffraction, UV–Vis spectroscopy, FTIR spectroscopy and FE-SEM techniques. FE-SEM images shows the formation of polyaniline nanofibers. The response of the prepared samples toward ammonia (NH3), ethanol and acetone gases was investigated by resistivity measurements of the samples at room temperature. Fabricated gas sensors have been showed a very good response toward NH3 (about 100%) but their response toward aceton and ethanol was negligible, which indicates high selectivity toward NH3 among these three gas mixture. In order to increase the sensitivity of samples, a thin laxyer (about 10 nm) of gold (Au) and nickel (Ni), as catalyst, was coated using vacuum thermal evaporation on top of PANi and PANI/ZnO composite. It was observed that the sensitivity of the sensors coated with gold was increased about three times for PANI and about two times for PANI/ZnO composite.