Ultra-high performance fiber-reinforced concrete (UHPC) is a new class of concrete that has been developed in recent years. UHPC results from the addition of either short discrete fibers or continuous long fibers to the cement based matrix. When UHPC compared with high performance concrete (HPC), UHPC exhibits superior properties in terms of compressive behavior, tensile behavior, workability, toughness, ductility and durability. UHPC 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 a total 42 cubes, 84 cylinders and 21 prisms of UHPC samples to study the effect of adding steel fibers on the mechanical properties of UHPC such as, compressive strength, modulus of elasticity, poisson’s ratio, flexural strength and tensile strength. The major parameters included in this research were the volume fraction of steel fibers and aspect ratio. The test results showed that the increase of volume fraction of steel fiber from 0% to 3% for UHPC causes maximum increase in compressive strength by 18.2%, flexural strength by 40% and tensile strength by 66.1% for higher side of aspect fiber ratio. Furthermore, adding steel fiber to UHPC can change the crack patterns, delay the crack appearance and restrain the crack expansion in concrete specimen.