The structures and stabilities of cage C 20 F 20 and its endohedral complexes X@C 20 F 20 (X=O 2− , S 2− , Se 2− ) were determined at the B3LYP/6-31G(d) levels of density functional theory (DFT). It is found that cage C 20 F 20 (I h ) possesses of a distinct C δ+ –F δ− spherical double electric layer, and the adiabatic electron affinity (EA ad ) of host cage is higher than that of isolated O atom (2.63 vs. 1.46eV). This suggests the C 20 F 20 cage can selectively trap and stabilize the capsulated spherical anions. The calculations predict that all guest species are minima at the cage center and the corresponding X@C 20 F 20 (I h ) complexes have negative inclusion energies (ΔE inc ) and thermodynamic parameters (ΔZ). The amount of charge that is being transferred from the encapsulated anions to the cage increases with the atomic radius, i.e., from O 2− (∼54%), S 2− (∼114%) to Se 2− (∼130%), and such a novel model of cage may have practical uses as potential and electrical building units of nanoscale materials.