A kind of NH3 gas sensors based on hydroxylated single-walled carbon nanotubes (SWCNTs-OH)/polyaniline (PANI) composites were fabricated by a simple in-situ chemical oxidative polymerization method. The microstructure characterization showed uniform PANI layer wrapped around the external walls of the SWCNTs-OH bundles, and the thickness of the PANI layer decreases with increasing of SWCNTs-OH content in composites. The FTIR and Raman analysis suggested that the presence of hydroxyl groups on SWCNTs-OH further enhanced the interaction between SWCNTs-OH and PANI via hydrogen bonding. The test results toward NH3 showed that response values of all SWCNTs-OH/PANI composites based sensors were higher than that of PANI. It could be attributed to the formation of a more massive conjugated structure due to the hydrogen-bonds between SWCNTs-OH and PANI, which enhanced spatial delocalization of p-type carrier holes in the composites, thus increased probability of NH3 molecules interacted with protons in SWCNTs-OH/PANI composites. However, the response values didn't increase all the time with increasing of SWCNTs-OH content in composites, and the SWCNTs-OH/PANI composites with 15 mg SWCNTs-OH based sensor exhibited the maximum response value when compared to the others. Its response value was about 50.03 times than that of the pure PANI based one at 800 ppm concentrations of NH3. It might be related to a balance between the number of active sites and the density of pores in composites, which both associated with the thickness of the wrapped PANI layer.