ESCA analysis identifies that the different functional groups of nitrogen compounds have been conbined on the PET surfaces, when modified by the air plasma and the argon plasma. In particular, the double N15 peaks occur on the PET surface after 5 minutes treatment by the low temperature argon plasma. The second peak-N15 is considered as the contribution of nitrogen oxides. The double peaks PET-N15 structure does not result by bombardment with low energy nitrogen ions and argon ions, which indicates that the effect can not be attributed to the action of low energy ion alone.