The excess electron binding mechanism of the anionic nitromethane-water clusters was theoretically investigated using the potential energy surfaces calculated by high-level electronic structure theories. The mechanism was first studied for the dipole-bound and valence-bound anionic states of the CH3NO2- monomer with the ab initio multireference configuration interaction method to reveal the electron transformation process between these anionic states in detail. As a result, it was found that both the NO2 tilting angle and NO distances play an essential role in this electron transformation. Following this result, various water solvation structures of the valence-bound CH3NO2- anion were optimized with up to six water solvents using the second-order Moller-Plesset (MP2) method. The calculated results predicted that the vertical detachment energy of the valence-bound CH3NO2- anion increases gradually with the hydration number, as is consistent with recent experimental observations. We also investigated metastable complexes composed of CH3NO2 and (H2O)(6)(-) by using the MP2 and long-range corrected density functional theory calculations. Two types of dipole-bound forms were obtained for the [CH3NO2 center dot(H2O)(6)] anion complex. In one form the excess electron is internally suspended between the two moieties while in the other form two dipolar moieties are cooperatively arranged to reinforce the electron-dipole interaction.