Density functional theory (DFT) was used to investigate the stability of Pa(V) and U(VI) oxocations in aqueous solution. As a result, DFT calculations clearly supported an experimental result from an energetic point of view that for Pa(V) in aqueous solution, the preferable species is not PaO2+ cation but PaO3+ cation. Calculated molecular orbitals indicated that 6d orbitals of Pa(V) destabilize the pi orbitals of PaO2+, because 6d-2p anti-bonding orbital conflicts with another 5f-2p bonding orbital. In contrast, UO22+ cation is a stable ion in aqueous solution. For this cation, we found that 6d orbitals of U(VI) forms 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,