The DielsAlder (DA) covalent dimerization mechanisms of pyridine were studied at the MP2/6-311G(d,p) level. Involvement of N atom in DA reaction causes high barrier height, otherwise the barrier heights are low. The two most stable DI dimers formed by intermolecular DA reaction (b-γDI and b-γ⿲DI) and the two most stable DII dimers formed by further intramolecular DA reaction (c-γDII and c-γ⿲DII) were located with relative energy of 26.6 and 26.1kcal/mol, and 32.1 and 31.6kcal/mol, respectively, higher than two moles of pyridines. Theoretical results suggested that DA dimerization of pyridine is slightly easier than that of benzene.