DFT calculations were used to study the cis → trans isomerization mechanism of 4,4′-DCA and 3,3′-DCA. We have detected two inversion transition states TS_Inv1 and TS_Inv2 for 4,4′-DCA and 3,3′-DCA, respectively. Calculation results show that substituents of carboxy groups have no obvious effect on the coplanarity of benzene rings and NN bond for trans isomers. Potential energy surfaces were calculated for 4,4′-DCA and 3,3′-DCA. In the ground state, the rotation-assisted inversion pathway was found to be preferred. A conical intersection was found between the ground and first excited states along the rotation pathway, indicating that the rotation mechanism is favored in the first excited state. We also analyzed two possible isomerization routes through the inversion and rotation pathways of 4,4′-DCA and 3,3′-DCA of S₂ states.