For crystallizable polymers such as polyethylene it is found that the mechanical response in a stress relaxation experiment is nonlinear, even at quite small deformations, and the behavior for some multistep stress relaxation strain histories cannot be described quantitatively by the BKZ theory. A better description of the behavior is obtained using a new constitutive equation proposed by Zapas. The predictions of both the BKZ theory and the new description are shown for a series of single and multistep stress‐relaxation experiments done in uniaxial extension on an ethylene–hexene copolymer. The results of the calculations using the new description are in good agreement with experiment.