We have studied the optimum deposition conditions for the improvement of oxygen and carbon dioxide gas barrier performance of functional diamond-like carbon (DLC) thin films. The a-C:H: a-C:H:Si, a-C:H:N and ta-C:N thin films with 10–400 nm thickness were deposited on polyethylene terephthalate (PET) substrates by the radio frequency plasma-enhanced chemical vapour deposition method. To study the microstructure of the PET coated films, we have used the Raman spectroscopy, X-ray photoelectron spectroscopy, nano-indentation and surface profilometry. The gas barrier property were analysed and found that the a-C:H:N's are 5–10 times better gas barrier properties than that of uncoated PET substrates. These thin layer PET coated thin films could be use in food packaging and biomedical applications. © 2017 Elsevier B.V.