Abstract: Based on the B3LYP density functional theory, the LANL2DZ basis set was employed to study the equilibrium geometries and vibration frequencies of two kinds of the isomers C50 with D5h/D3, symmetry and the clathrate M@C50 [M = Be, Mg, Ca, Sr, Ba] as the 2nd main group formed with the isomers as radicals. The energy level difference of frontier molecular orbital, net atomic charge and binding energy were calculated. The results indicated that the endofullerenes D5h M@C50 are more stable than the D3M@C50. For the same symmetry D5h, the stabilities of different endofullerenes are in order of their atomic number from low to high. Thermodynamic factors are thus quite favorable for the formation of Sr@C50 and Ba@C50.