The mechanism of reversible photoinduced anisotropy of an azobenzene derivative was investigated by measuring optical retardation and UV/Vis dichroism. During the relaxation period, the cis-trans thermal back relaxation and orientational relaxation of azobenzene groups occur. It is believed that the orientational relaxation of trans and cis azobenzene groups to relieve the strains is a more dominant contribution to the decrease of the optical anisotropy in the film.