The DFT/B3LYP method with different basis sets has been applied to the systems of (H2AlN3)(n) (n = 1-4). (H2AlN3)(2) is found to exhibit the planar Al2N2 ring structure and possesses D-2h symmetry. The transition state for the monomer to dimer conversion is located, and the activation energy is presented. (H2AlN3)(3) involving a six-membered Al3N3 ring is found to exhibit two minima with very similar binding energies (ca. -274 to -278 kJ(.)mol(-1)). One minimum found possesses C-s symmetry and displays boatlike conformation. Another minimum found possesses C-3nu symmetry with chairlike conformation. (H2AlN3)(4) occurs in several structures with Al4N4 eight-membered ring structures that correspond to the minima with slight energy differences among them. Compared to the monomer, both the structural changes and charge transfers for the clusters are large. Frequency calculations are carried out on each optimized structure, and their IR spectra are discussed. Thermodynamic properties reveal the system of H2AlN3 is involved in dimer-trimer-tetramer equilibria, and the trimer is the main component.