XeCl exoimer laser induced multiphoton ionization and fragmentation of NH3 are investigated, using a molecule-beam apparatus and modified quadrupble mass spectrometer. We have observed two mass peaks due to NH2+ and NH3+ at m/e=16 and 17, also found very strong mass peak of NH4+ at m/e=18. The yield dependences for various ions on the parent moleoular pressure and laser power were measured in order to have a better understanding of the generation mechanism for various ions, particularly NH4+ ion.