In order to investigate the efficiency of atmospheric pressure plasma jet （APPJ）treatment on PET films in terms of inducing free radicals and subsequent grafting polymerization, Electron Spinning Spectroscopy (ESR）technique is used in this paper to test and analyze the free radical outcomes and their aging phenomenon. The correlation between the ESR results and grafting ratio change trend for different samples is then discussed and analyzed. The results show that pure He gas APPJ is more easily to make free radicals generated on PET film and the free radical concentration is increased with the prolonged treatment duration. The stability of trapped free radicals on PET films varies obviously with different storage conditions. Samples with higher free radical concentration could also gain higher grafting ratio.