Photosensitive polymethacrylates having benzylidenephthalimidine(BPI) moiety on the side chains were synthesized. Upon photoirradiation of the polymer film, there occurred two kinds of photoreactions; the E/Z photoisomerization and [2+2] cycloaddition. The cycloaddition of BPI units of polymers resulted in the crosslinking of the polymer chains. The quantum yield for the photocrosslinking reaction was estimated from gelation experiment. Irradiation of the film with linearly polarized UV light induced a dichroism of BPI. Linearly polarized UV light irradiation of a nematic liquid crystals (LCs) cell assembled with a glass plate surface-modified with a Poly[N-(2-methacryloyloxy)ethyl-p-methoxy benzylidenephthalimidine] film brought about the homogeneous alignment.