The reaction mechanism of the Diels-Alder reaction of paramagnetic endohedral metallofullerene, La@C-82, and 1,2,3,4,5-pentamethylcyclopentadiene was studied theoretically and experimentally. Theoretical calculations revealed that this reaction proceeds via a concerted mechanism that includes formation of a stable intermediate. The activation energy of a retro-Diels-Alder reaction was also studied experimentally, which is in good agreement with theoretical results.