Plasma polymerization of benzonitrile was carried out with RF glow discharge system. The polymer was characterized by FT-IR, UV-VIS, and Elemental Analysis. the molecular weitht was determined by vapor pressure osmometer. for the first time the nonresonant optical Kerr effect(OKE) of plasma polymerized benzonitrile was measured, and a power law dependence of ${gamma}_{1111}$ on average polymerization degree was discussed. An ultrafast response was also observed.