Lithium-ion-containing fullerene Li⁺@C₆₀ has very high electron affinity and low-lying LUMO level at -4.3 eV due to encapsulated positive charged lithium ion in the fullerene cage. Because of this property, reactivity of Li⁺@C₆₀ on organic functionalization is very different from empty fullerene. In this presentation we will show the synthesis of lithium-ion-containing PCBM ([6,6]-phenyl-C₆₁-butyric acid methyl ester) (Li⁺@PCBM). The [5,6]- and [6,6]-isomers of [Li⁺@PCBM]PF₆^– were successfully prepared, and the structure of [6,6]-[Li+@PCBM]PF₆^– was elucidated by X-ray crystallography. In addition, we will discuss reactivity of the Diels-Alder reaction on Li⁺@C₆₀. The Diels–Alder (DA) reaction of fullerene is one of the most important reaction in the fullerene chemistry. We studied the reaction of Li⁺@C₆₀ with cyclopentadiene, which were chosen as an diene, and found that this reaction was highly efficient, with an equilibrium constant that was more than 1000-fold that for the reaction with empty C₆₀. The DA reaction of [Li⁺@C₆₀](PF₆^–) with 1,3-cyclohexadiene was also kinetically and computationally investigated to precisely determine the kinetic parameters. Compared with empty C₆₀, Li⁺@C₆₀ reacted 2400-fold faster at 303 K. The result was well explained by DFT calculation at the M06-2X/6-31G(d) level of theory considering the reactant complex with dispersion corrections as well as by the frontier molecular orbital theory.