The equilibrium structures, vibrational frequencies, and bond characteristics of NpO2(m+) ions and NpO2(H2O)n(m+) (m = 1-2, n = 1-6) complexes have been studied by carrying out ab initio calculations in the gas phase and aqueous solution. The geometries have been obtained at the B3LYP level with the use of the polarized continuum model (PCM). The computed structural parameters that are in reasonably good agreement with the available data show that the solvation effect leads to a red shift of the IR spectra and the weakness of interaction strengths in neptunyl ions. By comparing the structural properties and the density of states (DOS) of these aqua complexes in the gas phase and aqueous solution, it is found that the solvation effect can be simulated approximately with the calculations of these aqua complexes in the gas phase. In addition, the DOS of these aqua complexes together with the binding energies between the neptunyl ion and water molecule reveal that the penta-aqua complex is preferred for neptunyl ions in aqueous solution.