The purpose of this paper is to explore the UHPFRC time-dependent behavior under sustained tensile loading at very early age (3 days) and to highlight its sensitivity to the level of loading by laboratory testing and numerical analysis. Uniaxial creep and relaxation tests were performed at 3 days age at various stress levels by means of a Temperature-Stress Testing set-up (TSTM). Under low tensile stresses, the material exhibits a linear viscoelastic behavior. Under high tensile stresses the creep response becomes non-linear. A numerical algorithm was used to convert the creep results into relaxations and compare them with measured relaxations. A generalized Maxwell chain model was applied to predict the UHPFRC tensile relaxations at a low load level.