In order to investigate the mechanism of the N-H migration in asymmetrical metal-free porphyrins, four porphyrins of electron-withdrawing or electron-donating substituent at the β-position were studied theoretically. For porphyrin 2 (R = OMe), 3 (R = Me), and 4 (R = NO(2)), four different asynchronous N-H migration pathways exist due to symmetry reasons. The corresponding trans-, cis-, and transition state geometries were analyzed using a normal structure decomposition method. Our data show that the hydrogen migration of porphyrin 2, 3, and 4 in clockwise (A, B) are much more preferred than counterclockwise (C, D) direction.