Plasma-polymerized benzonitrile (PPBN) thin films were successfully prepared by radio frequency glow discharge techniques. The results of Fourier transform infrared (FTIR), UV-Visible absorption spectra, X-ray photoelectron spectroscopy (XPS) and on-line MS revealed that extensive conjugation has been formed in the PPBN thin films and the polymerization of benzonitrile took place mainly through the opening of carbon-nitrogen triple bonds. The morphology characterized by field emission microscopy (FEM) indicated that the fine, homogenous and brown transparent films could be obtained at comparatively low discharge power. Furthermore, the subpicosecond time-resolved optical Kerr effect (OKE) was used to measure the third-order optical nonlinearity. For the first time, an ultrafast response and nonresonant optical Kerr effect of PPBN films was observed.