Stress-dependence of the intrinsic time of viscoelastic materials was investigated in this paper. The influence of stress level on the intrinsic time is considered to be similar to that of temperature, pressure, solvent concentration, damage and physical aging. The time-temperature-stress equivalence principle is proposed. Using which, the creep curves at different temperatures and stress levels can be
shifted into a master curve at reference temperature and stress level. Thus the long-term creep behavior of viscoelastic materials at lower temperature and stress can be predicted from the short-term one at higher temperature and stress. As an example, the nonlinear creep behavior of high-density polyethylene (HDPE) at room temperature was studied using the presented time-temperature-stress  equivalence principle.