This study focuses on the SiOx:C thin films on polyethyleneterephtalate (PET) produced by radio-frequency (RF) magnetron sputtering with hexamethyldisiloxane (HMDSO). Silicon atoms are inserted into SiOx:C thin films using a silicon target to improve the gas barrier property. Since inserting Si atoms into SiOx:C thin films can reduce the number of pores and increase the density of the film. This process has two benefits. First, it can be easily combined with sputtering processes to produce a multilayer film; and second, it can be performed at low pressures (~10-1Pa) to improve the quality of the plasma polymer film. A more linear-like structure and a less cage-like structure is associated with higher density of SiOx:C thin films. The average transmittance of the films thus obtained exceeds 91% in the wavelengths of visible region, and the lowest water vapor transmission rate achieves 0.076g/m2/day. It is about one fourth of the optimum WVTR value (0.3g/m2/day) of a single layer of SiOx:C thin film fabricated with HMDSO at present.