The Diels-Alder (DA) covalent dimerization mechanisms of pyridine were studied at the MP216-311G(d,p) level. Involvement of N atom in DA reaction causes high barrier height, otherwise the barrier heights are low. The two most stable D-I dimers formed by intermolecular DA reaction (D-b-gamma(I) and D-b-gamma'(I)) and the two most stable D-I dimers formed by further intramolecular DA reaction (D-C-gamma(II) and D-C-gamma'(II)) were located with relative energy of 26.6 and 26.1 kcal/mol, and 32.1 and 31.6 kcal/mol, respectively, higher than two moles of pyridines. Theoretical results suggested that DA dimerization of pyridine is slightly easier than that of benzene. (C) 2015 Elsevier B.V. All rights reserved.