The true tensile and compressive stress-strain curves for a typical semicrystalline polymer (polypropylene) were constructed with a new experimental technique. This is based on a non-contact method of strain measurement along the specimen gauge length. The viscoplastic behaviour obtained in this manner was then described with a nonlinear viscoelastic constitutive model mentioned analytically elsewhere. This consideration leads to a satisfactory description of yield and post yield behaviour, including strain softening, and strain hardening. The rate effect was also predicted with a high accuracy.