HDPE pipes are frequently laid in buried or submerged conditions and are often subjected to considerable internal pressure. This context requires the consideration of HDPE as a structural material and demands constitutive models to predict failure possibilities in short and long terms. This article presents an approximate procedure to simulate the viscoelastoplastic nature of HDPE's material behavior under creep conditions. While a generalized Kelvin-Voigt model based on Prony series is used to model viscoelasticity, the power law of Zapas-Crissman is adopted to account for viscoplastic effects. The associated material parameters are obtained from experimental creep-recovery tests evaluated at different stress levels and constant temperature. As this type of test allows an uncoupled procedure for identifying the viscoelastic and viscoplastic material parameters, this task is divided into two stages: (i) a constrained nonsmooth optimization problem is defined and solved for the viscoelastic parameters, and (ii) the viscoplastic parameters are determined by linear regression. Thereafter, the viscoelastic and viscoplastic parameters obtained for each experimental stress level are interpolated linearly for intermediate stress conditions. Finally, a numerical-experimental example is presented, showing that the proposed procedure is able to reproduce adequately more complex loading conditions. POLYM. ENG. SCI., 57:144-152, 2017. (C) 2016 Society of Plastics Engineers