Ultra-high performance fiber-reinforced concrete (UHPFRC) is a new class of concrete that has been developed in recent years. UHPFRC results from the addition of either short discrete fibers or continuous long fibers to the cement based matrix. When UHPFRC compared with high performance concrete (HPC), UHPFRC exhibits superior properties in terms of compressive behavior, tensile behavior, workability, toughness, ductility and durability. UHPFRC has exceptional mechanical and transport properties including a very high tensile strength, strain hardening, and a density leading to a very low permeability. In this research, tests were carried out on twelve ultra high performance reinforced concrete beams with 100 mm × 200 mm cross section, 1600 mm total length to study the effects of adding steel fibers on flexural behavior of UHPC beams. The major parameters included in this research were the amount of internal steel reinforcement and the volume fraction of fibers. The test results showed that adding steel fibers to UHPC beams led to an increase in both load-carrying capacity and flexural ductility, also the concrete strain reaches to high percentage of its ultimate value that taking full advantage of the improved properties of UHPFRC, Furthermore adding steel fiber to UHPC beams can change the crack patterns, delay the crack appearance and restrain the crack expansion in concrete specimen.