The Diels-Alder reaction of cyclopentadinene (CP) with methyl acrylate (MA) in room-temperature ionic liquids (RTILs) is theoretically examined. In the present study, quantum molecular orbital theory is combined with a multicomponent reference interaction site model (RISM). Because RISM is free from statistical error, it is possible to overcome the serious difficulty in the description of the strong Coulombic interaction in RTILs. We focused on the origin of the relatively moderate solvation effects of RTILs and the mechanism of endo-exo selectivity.