In the present paper an effort is made to model the time-dependent behavior of high-density polyethylene (HDPE) with a one-dimensional integral representation. Owing to the plasto-viscoelastic behavior of the material, we assume that the total strain can be decomposed into a recoverable viscoelastic strain and an irrecoverable plastic strain. The viscoelastic deformation is represented by the Schapery thermodynamic theory. The plastic deformation is assumed to be accumulated during the loading history. An effective time concept is introduced for the plastic deformation, so that the response due to complex loading can be accounted for. The present representation gives a very good prediction of the responses of creep and recovery, two-step creep, and constant stress rate loading and unloading. It is also applied successfully to describe the process of preconditioning of semicrystalline polymers.