The stability of Pa(V) and U(VI) oxocations in aqueous solution were theoretically investigated by means of density functional theory calculations. As a result, the present calculations clearly supported an experimental result from an energetic point of view that monooxo protactinyl cation, PaO3+, is a preferable species for Pa(V) in aqueous solution, although dioxo protactinyl cation, PaO2+, is not a feasible form. By an analysis of molecular orbitals, we revealed that 6d orbitals of Pa(V) destabilize the d orbitals of PaO2+, because 6d-2p antibonding orbital conflicts with another 5f-2p bonding orbital. For stable dioxo uranyl cation, UO22+, we found that 6d orbitals of U( VI), in contrast, form a bonding orbital with the 2p orbitals, and this bonding orbital coexists at an angle with the 5f-2p bonding orbital due to an electron correlation.