The dynamic mechanical and birefringence behaviors of two kinds of polyethylenes, low- and medium-density polyethylenes having different thermal histories, were investigated over the ranges of frequency from 0.008 to 4.3 Hz and of temperature from 30 to 90°C. Reducing the behavior at various temperatures to a given reference temperature of 50°C, both behaviors showed rather broad frequency dispersions, corresponding to the α dispersion of the materials, with activation energies of the mechanical and optical relaxation processes of around 25 kcal/mol for every specimen tested. The behavior for a particular specimen of annealed low density polyethylene was further investigated by comparison with its frequency dispersion behavior of dynamic crystal orientation. The mechanical and birefringence dispersions were found to be definitely associated with the dynamic crystal orientation and could be assigned to an α1 mechanism arising from the so-called grain boundary phenomena, but not to an α2 mechanism resulting from a crystal disordering transition or pre-melting.The temperature dependence of the vertical shift factor for bringing the mechanical data to its master curve, bT(T,T0), was discussed in terms of Nagamatsu—Takemura’s proposal, which takes into account the volume effect of the crystalline region, and of Wada—Okano’s proposal, which takes into account the smearing out effect of the crystal lattice potential. Neither of the proposals gave any definite explanation of the temperature dependence nor any assignment of the mechanical dispersion. The temperature dependence of the vertical shift factor for optical data, pT(T,T0), was also discussed on the basis of a two-phase hypothesis, i.e., additivity of crystalline and noncrystalline optical contributions, with the conclusion that the dependence results not only from that of the fractional volume of each phase but also from that of the orientation compliance of the noncrystalline chain segments for the particular specimen of annealed low density polyethylene.