Photoreaction and photoinduced molecular reorientation in nanoscaled films of poly[oxy(4-n-alkyl-3,5-benzoate)oxy-1,4-phenylenediacryloyl] (PPDA-CnBZ), which contains n-alkyl side groups and photosensitive 1,4-phenylenediacryloyl (PDA) units in the main chain, were in detail investigated by linearly polarized infrared (IR) and ultraviolet (UV) absorption spectroscopy and optical retardation analysis. The PPDA-CnBZ polymer molecules were found to undergo mainly photodimerization upon UV light irradiation. However, the occurrence of trans-cis photoisomerization in the PPDA-CnBZ film could not be detected. In addition, irradiation with linearly polarized UV light (LPUVL) induced selective photoreaction of the PDA chromophores positioned parallel to the polarization direction of the LPUVL, which caused the molecular reorientation of the polymer chains. In the irradiated films the unreacted PDA chromophores were found to anisotropically distribute in a direction perpendicular to the polarization direction of the LPUVL, whereas the photodimerized PDA units displayed nearly isotropic distribution in the film plane. Thus, the LC alignment on irradiated PPDA-CnBZ films can be deduced from the relationship between the distributions of polymer chains and of liquid crystal (LC) molecules. In particular, the unreacted PDA chromophores remained at the film surface irradiated with LPUVL favorably align the LC molecules along their reorientation direction, which is perpendicular to the polarization direction of the LPUVL via their anisotropic interactions with the LC molecules.