By treating dosage, geometry and distribution of steel fiber as variables, the direct tensile behavior and bending toughness of ultra high performance concrete(UHPC) with the addition of deformed steel fibers were investigated. Moreover, the pull out channels and cross sections near the fractured surfaces of steel fibers were observed by means of microscope. The results show that deformed steel fibers have beneficial effects on the strengthening and toughening of UHPC, which indicates a significant improvement on the direct tensile strength, fracture energy and post cracking load bearing capacity of specimens incorporating steel fibers. Meanwhile, the improvement can be increased with the increasing dosage of steel fiber. Notably, steel fiber orientated along the direction of tensile stress provides a better performance with respect to the strengthening and toughening than the fibers randomly distributed. Furthermore, compared with hooked steel fiber, the influence of crimped steel fiber on strengthening and toughening of UHPC is more efficient and the pull out channels are more zigzag, due to the more pronounced mechanical interlocking between steel fiber and mortar matrix which further enhances the interfacial bond in between. In addition, noticeable necking phenomena appeared at cross section near the fractured surfaces of the two types of steel fiber during tensile test.