Polyethylene having 1,2- or 1,3-disubstituted cyclopentane structures was prepared by copolymerization of ethylene and cyclopentene (CPE) or 1,5-hexadiene (HD), and the crystalline structure was studied. Treatment with fuming nitric acid preferentially degraded the amorphous phase of the copolymers, and the structure of degraded samples indicated that 1,3-disubstituted cyclopentane units were taken in the crystalline phase of poly(ethylene-co-HD). Infrared spectra of the copolymers indicated the increase of hexagonal-phase character in poly(ethylene-co-HD) with increasing content of 1,3-disubstituted cyclopentane units. Wide-angle X-ray diffraction patterns of poly(ethylene-co-HD) showed a continuous transition from the orthorhombic phase to the hexagonal phase in the copolymer with HD content less than 15 mol %. On the other hand, 1,2-disubstituted cyclopentane structures were not taken in the crystalline phase, but in the amorphous phase.