Density functional calculations (B3LYP/6-31G*) have been carried out to investigate the transition state structure for the inversion mechanism in the thermal cis-trans isomerization 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 the cis and trans structures.