Endohedral metallofullerene has novel properties because of the interaction between the encapsulated metal atom or cation and fullerene. Experiments have demonstrated that the insertion of Li+ into C-60 can greatly promote the reactivity of the Diels-Alder (DA) cycloaddition of cyclopentadiene (CpH) to C-60. However, the reaction is sufficiently fast that its quantitative kinetic data cannot be obtained experimentally. In addition, knowledge regarding the effects of other alkali metal cations and metal cations with more charges on the reactivity and regioselectivity of C-60 is almost nonexistent. In the current study, DA cycloadditions of CpH to M+@C-60 (where M = Li, Na, K, Rb, and Cs) and Ca2+@C-60 were investigated via density functional theory in the gas phase and in solvent. Via careful discussion and comparison with the results of C-60, we concluded the following for the DA reaction of CpH to C-60 and, more generally, for DA reactions of other fullerenes: (1) the encapsulated metal cations enhance the reactivity; (2) among alkali metal cations, Na+ could be the best catalyst; (3) Ca2+ is more favorable in promoting the reactivity than any alkali metal cation; (4) encapsulated metal cations with more positive charges enhance the reactivity of the 6-5 bond in C-60, which is significant when the 6-5 adduct is the target product.