The electronic properties of single-walled carbon nanotubes (SWNTs) are sensitive to the gas molecules adsorbed on nanotube sidewalls. It is imperative to investigate the interaction between SWNTs and gas molecules in order to understand the mechanism of SWNT-based gas-sensing devices or the stability of individual SWNT-based field effect transistors (FETs). To avoid the Schottky barrier at the metal/SWNT contact, which dominates the performance of SWNT-based FETs, we utilize a contactless technique, dielectric force microscopy (DFM), to study the intrinsic interaction between SWNTs and gaseous ammonia molecules. Results show that gaseous ammonia affects the conductivity of semiconducting SWNTs but not metallic SWNTs. Semiconducting SWNTs, which are p-type doped in air, show suppressed hole concentration in ammonia gas and are even inverted to n-type doping in some cases.