Molecular motions in poly(ethylene-co-norbornene) were studied in a temperature range well below its glass transition point by a few techniques based on the NMR phenomenon. Temperature dependencies of proton spin-lattice relaxation times T₁ (at 200 MHz and at 30.2 MHz), proton spin-lattice relaxation time in the rotating frame T_1ρ (at 68 kHz) and frequency dispersion of proton spin-lattice off-resonance relaxation times in the rotating frame were determined for the copolymer. Analysis of ¹H NMR relaxation data permitted characterization of local motions occurring in the copolymer i.e. rotation of methyl groups around C₃ axes, reorientations of methylene groups and motions of segments of polymer chains including norbornene groups.