Summary: In this work, the large deformation behaviour under monotonic loading and unloading of a high density polyethylene (HDPE) is studied. To analyze the nonlinear time-dependent response of the material, mechanical tests were conducted at room temperature under constant true strain rates and stress relaxation conditions. A physically-based inelastic model written under finite strain formulation is proposed to describe the mechanical behaviour of HDPE. In the model, the inelastic mechanisms involve two parallel elements: a visco-hyperelastic network resistance acting in parallel with a viscoelastic – viscoplastic intermolecular resistance where the amorphous and crystalline phases are explicitly taken into consideration. The semicrystalline polymer is considered as a two-phase composite. The influence of the crystallinity on the loading and unloading behaviour is investigated. Numerical results are compared to experimental data. It is shown that the model is able to accurately reproduce the experimental observations corresponding to monotonic loading, unloading and stress relaxation behaviours at different strain levels. Finally, the model capabilities to capture cyclic loading-unloading behaviour up to large strains are discussed. To demonstrate the improved modelling capabilities, simulations are also performed using the original model of Boyce et al. [M. C. Boyce, S. Socrate and P. G. Llana, “Constitutive model for the finite deformation stress-strain behavior of poly(ethylene terephthalate) above the glass transition”, Polymer 41, No. 6, 2183–2201 (2000; doi:10.1016/S0032-3861(99)00406-1)] modified by S. Ahzi et al. [“Modeling of deformation behavior and strain-induced crystallization in poly(ethylene terephthalate) above the glass transition temperature”, Mech. Mat. 35, No. 12, 1139–1148 (2003; doi:10.1016/S0167-6636(03)00004-8)].