The mechanical relaxation behaviors of oriented samples of a linear low-density polyethylene (LLDPE) and a high-density polyethylene (HDPE) were studied using tensile dynamic mechanical measurements. The anisotropy and activation energies of the relaxations were determined for several different samples to investigate why the α-relaxation in HDPE has characteristics similar in some respects to both the α- and β-relaxations in LLDPE. It is concluded that the anisotropy of the α-relaxation in LLDPE is determined byc-shear, whereas in HDPE it relates to interlamellar shear. The activation energy measurements, however, show that the thermally activated process for the α-relaxation in both the LLDPE and HDPE isc-shear. It is proposed that, in HDPE,c-shear has to occur before there is enough mobility at the fold surface for interlamellar shear. It is concluded that the β-relaxation is not observed in HDPE because the interlamellar regions are too constrained to allow interlamellar shear withoutc-shear.