The time-dependent deformation behaviour of a commercial grade of high-performance polyethylene fibres (Dyneema SK66), was studied using static and dynamic mechanical testing. A mathematical model is proposed, where the total deformation of the fibre is regarded as being composed of a stress-linear delayed elastic component and a non-linear plastic flow contribution. The thermo-rheological characteristics of the delayed elastic contribution are obtained using dynamic mechanical thermal analysis. The non-linear plastic flow component is characterized separately using long-term creep experiments. Model predictions of stress relaxation and tensile experiments are in good agreement with the experimental data.