The PTFE films irradiated by Ar plasma were studied by using the method of ESR spectrometer. The peroxide free radicals were found on the surface of PTFE after plasma irradiation, which existed in the layer of some thickness of the surface. These free radicals were quite stable at room temperature. But when density of peroxide free radicals on the PTFE surface was big enough, terminational reaction between the peroxide free radicals cound happen and endoperoxides were formed on the PTFE surface. Also the density of peroxide free radical decreased with increasing temperature and underwent a sudden change with temperature reaching 399 K— the glass transition temperature of PTFE. The peroxide free radicals can be caught by free radical catcher PBN to form a stable free radical, also can react with MMA to form grafting layer.