Methyl methacrylate monomer was graft-copolymerized on a polyethylene (PE) surface and the tethered poly(methyl methacrylate) (PMMA) chains were spin-labelled to study the molecular motion of the PMMA chains by electron spin resonance (e.s.r.).Molecular mobility of the PMMA chains is strongly affected by physical structures of both PE and PMMA phases. For example, the transition temperature of molecular motion, T-5.0mT, at which the extreme separation width due to N-14 anisotropic hyperfine splitting is 5.0 mT, increases with increases in the grafting ratio and the crystallinity of the PE phase. The transition temperature for the grafted sample is lower than that for PMMA homopolymer bulk and the mobility of the PMMA chains in the mobile region of the grafted sample is much higher than that of PE chains on the PE surface. It is concluded that the structure and molecular motion of the PMMA chains tethered on the PE surface are strongly affected by the PE chains in the amorphous phase, the structure and molecular motion of which are also closely related to the crystallinity and/or the size of crystalline PE.