Unidirectional liquid crystal (LC) alignment by a linearly polarized UV exposure was examined using various polyimides (PIs) which consist of different tetracarboxylic dianhydrides and diamines. Measurement of dichroic ratios of LC cells suggests that sensitivity of unidirectional LC alignment for UV dosage is largely affected by the chemical structures of PIs used. The results of dichroic ratio measurement of LC cells, UV absorption spectra, birefringences, FT-IR spectra of PI films, and molecular conformations of diamines in Pis calculations using molecular mechanics suggest that the selective photo-decomposition of PI caused the anisotropic van der Waals forces which aligns the LC along its optic axis in residual PI chain.