Photochemical phase transition of polymer liquid crystals (LCs) with hydoxyazobenzene moieties has been studied. By irradiation with the laser pulse at 355 nm, photochemical LC-isotropic (I) phase transition of the polymer LCs was induced in microsecond timescale. When the irradiated sample was kept in the dark, the initial LC phase recovered thermally. The response time of the I-LC phase transition was 300 Pas in hydoxyazobenzene LCs, while it was 1.5 s in O-alkyl analogues. In the cis-trans isomerization process in hydroxyazobenzenes, the hydrazone was produced by proton transfer, and this tautomer is converted into trans-azobenzenes. In the thermal cis-trans isomerization, hydroxyazobenzenes prefer this two-step mechanism to the one-step mechanism because the activation energy of this mechanism is lower than that of the one-step mechanism. Thus, the thermal isomerization occurred effectively, and the recovery of the LC phase took place quickly in the present polymer LC system. The results obtained here suggest an approach to accelerate the thermal recovery of the LC phase.