Structures of unprotonated (NH3)(n)(+) (n = 1-6), protonated NH4+(NH3)(n-1) (n = 1-6), and proton-transferred (NH4+-NH2)(NH3)(n-2) (n = 3 - 7)] ammonia cluster cations have been optimized with ab initio Hartree-Fock (HF) and second-order Molle-Plesset (MP2)/6-311-G** levels, and the harmonic vibrational frequencies have also been evaluated. In ammonia cluster cations, NH3+, NH4+, and NH4+ - NH2 form as a central core of the first ammonia solvation shell, respectively. In unprotonated dimer cation, the totally symmetric structure with the head-to-head interaction is optimized to be stable. In the hydrogen-bonded dimer cation, the unprotonated NH3... NH3+ cation is optimized to the protonated-transferred NH4+... NH2 cation. In unprotonated trimer cation, there are two types of isomers (hydrogen-bonded and head-to-head). The hydrogen-bonded type of unprotonated trimer cation is more stable. In unprotonated pentamers and hexamers, a NH3+ core has both interactions in a complex. On the other hand, in unprotonated tetramer, protonated, and proton-transferred cations, a core has only the hydrogen-bonded interaction. In unprotonated tetramer and protonated cations, isomers do not exist, while, in the proton-transferred cations, some types of isomers are optimized at the MP2 level. With increasing cluster cation size, the bond lengths [R-NN] between two nitrogen atoms and the distances [R-N ... H] of the hydrogen bond increase regularly. The binding energies (E-n,(n-1)) of ammonia cluster cations decrease with size. For the dissociation reaction of unprotonated heptamer (NH3)(7)(+) into protonated tetramers NH4+(NH3)(3) and NH2(NH3)(2), the proton-transferred heptamer (NH4+ - NH2)(NH3)(5) is the most stable one.