Hydrogenated carbon thin films were fabricated on the surface of polyethylene terepthalate(PET) by radio frequency plasma enhanced chemical vapor deposition (r. f.PECVD).The film structure properties were studied by means of atomic force mic roscope(AFM), x_ray photo_electron (XPS), laser Raman spectroscopy, Fourier_tra nsform infrared spectra(FTIR),etc. The barrier property of the film was conducte d on the water vapor permeation instrument. The results show that nano_hydrogen ated carbon films have been deposited on PET surface and they are mainly compose d of sp2 and sp3 hybridized hydrogenated carbon compounds. Plasma pa rameters influence the films' growth rate and structure characteristics. The fil m reduces the water vapor permeation ratio of the PET by 7 times at a film thick ness of only 900nm.