We performed a theoretical investigation for the selectivity of Eu(III)/Am(III) ions depending on the donor atoms by means of all-electron ZORA-DFT calculation. We estimated their selectivity as the relative stability in the complex formation reaction. The B2PLYP functional reproduced the experimental selectivity in which S- and N-donor ligands favor Am(III) ion, but O-donor ligand favors Eu(III) ion. Mulliken's bond overlap population analysis revealed that the contribution of the f orbital to the bonding was small or zero for Eu complex, whereas it was large for Am complex. The bonding nature of the f orbital for Am ion was the bonding type to S- and N-donor ligands, while it was the antibonding type to O-donor ligand. It was suggested that the difference in the bonding nature between the f orbital in the metal and the donor atoms determines the selectivity of Eu(III)/Am(III) by donor ligands.