We used a combined quantum mechanical/mechanical molecular (QM/MM) method to investigate ionization and electron attachment for nucleobases in water. We first explored the snapshot number and QM-region size to get converged results. The computations of 20 snapshots can get converged physical quantities and converged QM regions contain about 100 atoms. We computed the vertical ionization energies (VIEs), adiabatic ionization energies, vertical electron affinities, adiabatic electron affinities, and vertical detachment energies of five bases in the explicit aqueous solvent. The computational VIEs of cytosine and thymine agree well with the experimental values in aqueous solutions. The results of the five physical quantities indicate that the QM-region polarization by bulk water evidently affects the ionization and electron attachment. Holes or excess electrons do not entirely localize on nucleobases in the ionization and electron attachment. The QM-region polarization and the movements of the QM atoms induce a drastic decrease (∼2.1 eV for cation and ∼2.9 eV for anion) of the cationic and anionic energies in structural relaxations. The potential energy surfaces of the cation and anion would become very steep near the equilibrium structure.