Photochemical phase transition behavior of polymer liquid crystals (LCs) with donor-acceptor azobenzene moieties which contain both electron-donating and -accepting substituents at the 4,4＇-positions has been explored. Polymer azobenzene LCs were prepared which show nematic (N) LC behavior in the trans form but no LC phase in the cis isomer. Photoirradiation of a thin film of the azobenzene LCs (similar to 200 nm) in the trans form resulted in disappearance of the N phase due to trans-cis photoisomerization of each mesogen, and the initial N phase was recovered when the irradiated sample was kept in the dark because of cis-trans thermal isomerization and reorientation of trans-azobenzenes. Time-resolved measurements with a laser pulse (355 nm; 10 ns, fwhm) revealed that the N-to-isotropic (I) phase transition took place in 200 mu s. The thermal I-N phase transition of a donor-acceptor azobenzene LC with an acrylate main chain occurred in 800 ms at 135 degrees C. This response is faster by one order of magnitude than the response of non-donor-acceptor azobenzene LCs. In the sample with a methacrylate main chain, however, the thermal recovery of the N phase took place slowly. These phenomena are interpreted in terms of the mobility of segments of the polymer.