Density functional calculations (B3LYP/6–31G*) have been carried out to investigate the transition state structure for the inversion mechanism in the thermalcis-transisomerization reaction of azobenzene and 4-amino-4′-nitroazobenzene. Four and eight conformations were optimized in order to find the inversion transition state of azobenzene and 4-amino-4′-nitroazobenzene, respectively. As a result, it is found that azobenzene and 4-amino-4′-nitroazobenzene have only one genuine inversion transition state, respectively. The inversion barrier of 4-amino-4′-nitroazobenzene is smaller than that of azobenzene. The dipole moment of the inversion transition state of 4-amino-4′-nitroazobenzene is extremely large compared to those of thecisandtransstructures.