Eight relaxation processes have been investigated below the melting point in high-density polyethylene (HDPE) by relaxation spectrometry, and the nature of the processes has been examined. Two relaxation processes ( β and β 1) correspond to small-scale motion of the polymer chains in amorphous and crystalline phases. Three relaxation processes ( α, α 1 and α 2) correspond to segmental mobility of the chains respectively in amorphous phase (glass transition process, in crystal→amorphous phase transition layers, and in amorphous parts of spherulite fibrils. The three high-temperature processes ( λ 1, λ 2 and λ 3) relate to molecular mobility of elements of supersegmental structure of HDPE amorphous phase and are similar to those in noncrystalline polymers that are in the high-elastic state (elastomers).