Zinc oxide (ZnO) nanoparticles were synthesized by simple route of sol-gel method and nanofibers of polyaniline (PANI) and PANI/ZnO nanocomposites prepared using the electrospinning technique. Electrospun nanofibers of PANI and PANI/ZnO nanocomposites were collected on aluminum substrate for characterization and on Cu-interdigited electrodes to prepare chemiresistor sensor. Electrospun nanofibers of PANI and PANI/ZnO nanocomposites have been characterized by UV-Visible (UV-Vis), Fourier transform infrared (FTIR), X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). UV-Visible spectra of nanofibers of PANI/ZnO nanocomposites show hypsochromic shift as that of PANI and some interaction due to ZnO in PANI matrix. The observed changes in the FTIR spectra of the fibers of PANI/ZnO nanocomposites are assosciated with the formation of H-bonding between ZnO and the N-H group present in the PANI chains. X-ray diffraction patterns exhibits hexagonal wurtzite structure of ZnO and broad amorphous peaks of PANI. Heterogeneous structures with fibrous characteristics of diameter less than 300nm of PANI/ZnO nanocomposites are identified in the SEM images. The electrical properties were characterized by I-V characteristic measurements. The changes in resistance of the chemiresistor sensor were utilized for detection of HCl and NH3 chemical vapour at room temperature. The resistance of the sensors was found to be decreased when they were exposed to HCl vapours whereas the resistance of the sensors was found to be increased when they were exposed to NH3 vapours. It was observed that PANI/ZnO nanocomposite sensor shows a high response and sensitivity with good repeatability as compared to that of pure PANI. Sensitivity result shows that PANI/ZnO nanocomposite is highly sensitive to chemical vapours even at room temperature and at very low concentration.