Abstract: Multiphoton Ionization （MPI） of ammonia clusters produced in a supersonic molecular beam has been carried out in laser wavelengths of 427-442 nm. The cluster ions measured by TOF mass spectrometer contained N atom numbers up to 17. Aside from protonated （NH₃）_nH⁺, we have observed unprotonated （NH₃）_n⁺, and other two kinds of ions （NH₃）_nH₂⁺ and （NH₃）_n-1NH₂⁺ as well. The mass spectrum is more closc to that of electron impact rather than that of UV MPI. Ion intensity distribution is somewhat different at shorter and longer wavelengths. Possible mechanism of the cluster ion formation is discussed.