This paper investigates experimentally the complete uniaxial tensile stress-strain relationships of Ultra High Performance Concrete (UHPC) and equations for expressing such relationships are obtained. The effects of two variable parameters on the uniaxial tensile behavior are carefully studied which are the silica fume content SF (0%, 10%, 15%, 20%, 25%, and 30%) as a partial replacement by weight of cement, steel fibers volume fraction Vf (0%, 1%, 2% and 3%). The experimental results showed that the two variables of the present investigation have no significant effect on the shape of the ascending part of the stress-strain curve, while the descending part of the curve is found to be considerably affected by the volume fraction Vf of the steel fibers used. Increasing fibers volume fraction Vf from 0% to 3% not only increases the area under the uniaxial tensile stress–strain curve, but also increases
both the maximum uniaxial tensile strength by 238.5% and its corresponding strain by about 4044%. Two nonlinear equations are suggested in this research to model the ascending and descending part of the uniaxial tensile stress-strain relationship which are found suitable to represent the uniaxial tensile behavior of the UHPC mixes.