[reaction: see text] An intermolecular cobalt-phosphine-catalyzed Diels-Alder reaction of piperine is described. The dimerization of piperine in the presence of cobalt-phosphine complexes gave much better yields than that in the presence of only cobalt, which, combined with the result under the purely thermal conditions, indicates that addition of phosphine ligands changes the inhibition of cobalt to the reaction into promotion. For elucidation of the distinction, different cobalt-catalyzed mechanisms were proposed for the Diels-Alder dimerization of piperine.