Molecular structures, electronic spectra, and the thermal trans-cis isomerization of azobezene (AB) confined inside an armchair (8,8) single-walled carbon nanotube (CNT(8,8)) were calculated at the ONIOM(B3LYP/6-31+G*:UFF) level.We found that the entrance of azobenzene into CNT(8,8) was exothermic. The geometric parameters of azobenzene were not evidently affected by its confinement in CNT(8,8). Rotation of the phenyl rings was found to occur around the CN bond to some extent for both trans-and cis-azobenzene, which resulted in a twist configuration for the confined trans-azobenzene. The relative energy between the cis-and trans-azobenzene confined inside the CNT (8,8) increased by ca 8.1 kJ·mol-1 with respect to the case of the isolated azobenzene, suggesting that the relative thermal stability of the isomers of azobenzene is affected by their confinement in CNT(8,8). Electronic spectrum calculations showed that the lowest three singlet excitation energies of the confined azobenzene were blue-shifted by 1-5 nm in comparison to those of isolated azobenzene. By analyzing the potential energy surfaces, we found that the confinement in CNT(8,8) resulted in the increase in the activation barrier of the trans-to-cis isomerization of azobenzene but had little influence on the barrier of cis-to-trans backward reaction, which means that the confinement in CNT(8,8) restrained the trans-to-cis isomerization of azobenzene. The trans-cis isomerization process of the confined azobenzene mainly involved the bending of the CNN bond angle.