The linear theory of viscoelasticity has been well formulated and successfully used in the past. In comparison, however, nonlinear viscoelastic plastic behavior at finite strains is less understood because of several complexities characteristic of polymer solids. In order to construct a visco-elastic-plastic constitutive equation in polymer solids, systematic experiments should be planned to demonstrate the effects of time, temperature and pressure on their material properties. In this paper, compressive and torsional tests were performed using polyethylene (PE) specimens under the conditions of constant strain rate, abrupt change in strain rate, stress relaxation and creep to provide some experimental facts on the time dependence of stress-strain curves. The experimental results were compared with the numerical results based on a over-stress theory. It is shown that the over-stress theory well explains the stress-strain curves of PE in the case where the current strain is not below the previous strain and the theory may become a valid model to infer the nonlinear behavior not only at a uniaxial stress state, but also at a biaxial one.