Short- and long-term tensile creep tests of high-density polyethylene (HDPE) have been performed at different stress levels under an ambient temperature of 20 degrees C. The effects of stress and physical ageing on the creep compliance are studied. The short-term creep data show that the distribution of relaxation times of HDPE is shifted even by a very low stress. Acceleration of creep can be realized by applying high stresses, and a momentary master curve of creep compliance can be constructed using the time-stress superposition principle. At low stresses, though the material exhibits strong non-linearity, the ageing rate mu is found to be independent of stress. A unified non-linear creep relation is obtained that incorporates the physical ageing effect and predicts the long-term creep behaviour with good agreement with experiments.