Photo- and thermal isomerization of poly[2-(2-methacryloyoxy)azobenzene] derivatives was investigated both in solution and in film state. In the photo-induced trans-cis isomerization, the rate in film state was smaller than that in solution. The kinetics of the thermal back reaction was proved to depend on the copolymerization ratio of the polymer. Linearly polarized light (LPL) irradiation of a thin film of the polymer induced dichroism of the film. The degree of the dichroism was related to the Tg of the polymers. In-plane alignment of nematic liquid crystals (LCs) was regulated by photodichroic thin films. Heat treatment of a film before irradiation was a important factor for the photoregulation.