The pyrolysis mechanisms of quinoline and isoquinoline were investigated using the density functional theory of quantum chemistry, including eight reaction paths and a common tautomeric intermediate 1-indene imine. It is concluded that the conformational tautomerism of the intermediate decides the pyrolysis products (C6H6, HC C-C N, C6H5C N and HC CH) to be the same, and also decides the total disappearance rates of the reactants to be the same, for both original reactants quinoline and isoquinoline during the pyrolysis reaction. The results indicate that the intramolecular hydrogen migration is an important reaction step, which often appears in the paths of the pyrolysis mechanism. The activation energies of the rate determining steps are obtained. The calculated results are in good agreement with the experimental results.