We have investigated the photo-induced in-plane anisotropy of the films of two different polyimides (PIs) containing azobenzene in the backbone structure (Azo-PIs). which were synthesized from the same diamine, 4, 4'-diaminoazobenzene (DAAB), and two different dianhydrides, pyromellitic dianhydride (PMDA) or oxydiphthalic anhydride (ODPA). Under the same photo-alignment condition, the in-plane anisotropy of the ODPA-DAAB PI film was significantly larger than that of the PMDA-DAAB PI film. This result shows that the photo-alignment efficiency of the former is higher than that of the latter; i.e. the photo-alignment efficiency of Azo-PI can be improved by changing the backbone structure. We believe that the observed enhancement in the photo-alignment efficiency is attributed to a lowering of the polymer chain rigidity, which is caused by introducing an ether linkage with free rotation around non-parallel bonds into the dianhydride part. (C) 2009 Elsevier B.V. All rights reserved.