Hydrogenation of poly(vinyl cinnamate) was performed with RhH(CO)(PPh3)(3) catalyst with intent to clarify the photoalignment mechanism of liquid crystals on poly(vinyl cinnamate) alignment layer. The hydrogenated cinnamate side group shows the characteristic peaks at 2.9 ppm and 2.5 ppm in H-1-NMR spectrum, and a decreasing peak at 280 nm in UV spectrum. This reaction can reveal the role of dimerized or residual cinnamate groups in the applications of the polymeric layer fur electronic devices.