Experimental observations have shown that most of plant fibres are characterised by an intricate structure, morphology and organisation. This complex geometrical characteristics may affect the mechanical behaviour of this kind of natural fibres and contribute to the high variability of their mechanical properties. So, this study proposes a numerical investigation on the relationship between the cross-sectional shape of primary hemp bast fibres and their tensile behaviour. A 3D viscoelastic model based on finite element method is used for this study. Real and elliptical simplified cross-sectional shapes are considered. Results of the tensile test simulations clearly show the strong influence of the degree of ellipticity on the tensile response of the fibre, and more exactly on the shape of the nonlinearity of the response. Results also show that this morphologic effect is strongly related and coupled to structural parameters and physical mechanisms, such as the cellulose microfibrils angle and the viscoelastic behaviour of the material of the fibre wall. Geometric issues could then contribute to explain the different types of tensile behaviour experimentally observed and deserve to be taken into account in plant fibre models.