The potential energy surfaces for intramolecular hetero Diels−Alder cycloaddition reactions have been characterized in detail using the B3LYP density functional method, including zero-point corrections. Fifteen isoelectronic species (YCHX(CH2)4CHCHCHCH2) were selected as model systems to investigate both the energetics of the reactions and the geometric structures of the species involved. The systems can be divided into three groups as follows: (I) X = N and Y = CN, OH, H, CH3, Li; (II) Y = CB and X = N, P, As, Sb, Bi; (III) Y = CN and X = CH, SiH, GeH, SnH, PbH. The main findings are as follows. (1) The intramolecular hetero Diels−Alder reactions are predicted to take place through concerted but highly asynchronous transition structures in a fashion similar to some intermolecular hetero Diels−Alder reactions. (2) Our theoretical results for the geometries of the cycloadducts are in accordance with the experimental observations of Danheiser et al. (3) The substitution of a heavy heteroatom (i.e., X = SiH, GeH, SnH, PbH, P, As, Sb, Bi) for a nitrogen in the iminoacetonitrile exerts an important influence on both the activation energies of the transition structures and the reaction enthalpies of the cycloadducts.