Ultra High Performances Fibre Reinforced Concretes (UHPFRC) are characterised by an extremely low permeability and outstanding mechanical properties. These characteristics make UHPFRC suitable for locally “harden” reinforced concrete structures subjected to aggressive environments and/or mechanical stresses. Results from uni-axial tensile tests on different UHPFRC materials and cast in different directions have shown a variety of mechanical behaviour. In particular, the hardening behaviour ranges from 0.1-0.4 % but is, in some cases, not existent. A meso-mechanical model is developed to predict the UHPFRC tensile response as a function of the volume, aspect-ratio, distribution and orientation of the fibres and the mechanical properties of the matrix. The model allows determining the effect of two parameters, the coefficient of orientation and the volume of fibre, on the hardening behaviour.