Ultra High Performances Fibre Reinforced Concretes (UHPFRC) are characterised by an ex-tremely 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 behav-iour. 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 coeffi-cient of orientation and the volume of fibre, on the hardening behaviour.