Two Er@C-82 isomers have been isolated and unambiguously characterized as Er@C-2v(9)-C-82 and Er@C-s(6)-C-82, respectively, by single-crystal X-ray diffraction. Er@C-s(6)-C-82 is identified as a dimeric structure in the crystalline state, but dimerization does not occur for Er@C-2v(9)-C-82 under identical crystallization conditions, indicating a cage-symmetry-induced dimerization process. Density functional theory calculations reveal that the major unpaired spin resides on a special C atom of Er@C-s(6)-C-82, which leads to regioselective dimerization. Calculations also found that the dimeric structure of Er@C-s(6)-C-82 center dot Ni(OEP) is much more stable than the two monomers, suggesting a thermodynamically favorable dimerization process. Vis-near-IR spectrometric and electrochemical results demonstrate that the electronic structure of Er@C-82 isomers is Er3+@C-82(3-), instead of the theoretically proposed Er2+@C-82(2-).