Chain end peroxy radicals of polyethy!ene (PE) molecules tethered on a fresh surface of poly(tetrafluoroethylene) (PTFE) were produced in the following way. Ethylene monomer was block copolymerized by initiation of mechanoradicals formed by mechanical fracture of the PTFE powder. The propagating radical in the block copolymerization, -CH2CH2., located on the fresh surface of PTFE was converted into peroxy radicals, -CH2CH2OO.’, after introduction of oxygen molecules. Temperature-dependent ESR spectra of the peroxy radicals were successfully observed. The remarkable change with anisotropic g-values of the radicals was analyzed by computer simulation, and it was found that the PE peroxy radicals on the PTFE surface have extremely high mobility in comparison with the PE radicals on the PE surface and the PTFE radicals on the PTFE surface. This is a reflection of the properties of the protruding PE chains on the PTFE surface; i.e., interaction between PE chains from the PTFE surface must be much weaker than those on the surface of the homopolymer, because of the extremely low segmental concentration of the PE molecules on the PTFE surface.