Abstarct: In this research we have studied the morphological, structural, optical, and gas sensing properties of polyaniline nanofiber thin films, polyaniline/nickel ion and polyaniline/nickel oxide nanocomposites grown on glass substrates by spin coating method. Nickel oxide nanoparticles were synthesized using hydrothermal method. The samples were characterized by field effect scanning electron microscopy (FESEM), X-ray diffraction (XRD), UV-Vis spectroscopy and fourier transform infrared (FTIR) spectroscopy. In this research, the studied samples were prepared using aniline, nickel chloride and nickel oxide precursors. Polyaniline/nickel ion and polyaniline/nickel oxide nanocomposites were prepared using different percentages of nickel chloride and nickel oxide, respectively. The FESEM images of the samples indicates the formation of the nanofibers with different diameters. The XRD pattern reveals the amorphous nature of the polyaniline and polyaniline/nickel ion nanocomposite and hexagonal structure for nickel oxide and polyaniline/nickel oxide nanocomposites. The sensing response of the prepared samples to ammonia (NH3), ethanol and acetone gases was investigated by resistivity measurements at room temperature. The fabricated gas sensors have been showed a very good response to NH3 but their response to aceton and ethanol was negligible. This indicates high selectivity to NH3 vapor compared to other two gases. Maximum sensitivities of polyaniline, polyaniline/nickel ion and polyaniline/nickel oxide to the 100 ppm ammonia gas were about 34, 74 and 134 %, respectively.