Abstract: Structures of many endohedral metallofullerenes (EMF) are still unknown because of their very low production yield, especially for endohedral mono-metallofullerenes below C70. In the present work, we investigated the alkaline earth metal encapsulated mono-metallofullerenes M@C66 (M＝Ca, Sr, Ba) theoretically, compounds which have been detected only in mass spectra. The first-principles calculations and statistical thermodynamic analysis indicate that among the C66 isomers considered, the cage C2v(4348)-C66 should be the most likely structure for M@C66, M＝Ca, Sr, Ba. These compounds may adopt another cage structure, C2(4466)-C66, at high temperatures. Both M@C2v(4384)-C66 and M@C2(4466)-C66 possess relatively large HOMO-LUMO gaps, and the former also possesses high local aromaticity. Thus it is predicted that M@C2v(4348)-C66—especially Ca@C2v(4348)-C66—could be isolable in future experiments. Moreover, simulated Vis-NIR, IR, Raman and 3C NMR spectra are provided for assisting future experimental characterizations.