The authors have studied the Diels-Alder reactions between furan derivatives and maleimide derivatives in an ionic liquid and have found that higher reactivity can be obtained in a protic ionic liquid [Mim]Tf2N than in the conventional organic solvent. Furthermore, in the Diels-Alder reactions of 2- and 2,5-alkylfurans with N-alkylma-leimide, the reactivity increases by extending the alkyl chain length of N-alkylmaleimide. In addition, it was demon-strated that endo-selectivity increases when 2,5-disubstituted furans are used. These results will be explained by comparing the stability of the Diels-Alder adduct with that of the products obtained from the reactions of 2-substituted furans and 2,5-disubstituted furans. The authors have studied the Diels-Alder reactions between furan derivatives and maleimide derivatives in an ionic liquid and have found that higher reactivity can be obtained in a protic ionic liquid [Mim] Tf2N than in the conventional organic solvent. Further, in the Diels- Alder reactions of 2- and 2,5-alkylfurans with N-alkylma-leimide, the reactivity increases by extending the alkyl chain length of N-alkylmaleimide. In addition, it was demon-strated that endo-selectivity increases when 2,5- distributed furans are used. These results will be explained by the stability of the Diels-Alder adduct with that of the products obtained from the reactions of 2-substituted furans and 2,5-disubstituted furans.