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 different gases. In this thesis, gas sensors were fabricated on the basis of polyaniline nanofibers, polyaniline/zinc chloride and polyaniline/tin chloride nanocomposite on glass substrates using spin coating technique. The method of synthesizing the polyaniline nanofibers and its nanocomposites was baxsed on in-situ chemical polymerization. The weight percentages of zinc chloride and tin chloride in the nanocomposites were choose to be 0.2, 0.5, 1, and 1.5 wt%. Structural and optical properties, composition and surface morphology of prepared samples were investigated using X-ray diffraction (XRD), UV-Vis spectroscopy, fourier transform infrared spectroscopy (FTIR) and field effect scanning electron microscopy (FESEM), respectively. FESEM images show the formation of polyaniline nanofibres. Mean diameter of these nanofibers for polyaniline was estimated to be about 40 nm. X-ray diffraction pattern analysis revealed amorphous structure for polyaniline and its nanocomposites with zinc and tin chlorides. The sensitivity (gas response), response time and recovery time of samples were measured at room temperature and toward ammonia gas using conductometery method. Fabricated sensors showed a good sensitivity to ammonia gas, so that maximum sensitivity for polyaniline, polyaniline/zinc chloride nanocomposite and polyaniline/tin chloride was reached to about 25, 57 and 93%, respectively, to ward 100 ppm of ammonia. The best sample in terms of sensitivity, response time and recovery time toward 100 ppm of ammonia gas was 1 wt% polyaniline/tin chloride nanocomposite. The sensitivity of the samples to blue light (wavelength ~ 490 nm) was also investigated which indicates that the samples are sensetive to light blue, which emphasizes the optical sensing of the compounds mentioned.