Abstract: In order to investigate the effect of short and long steel fibers on the flexural response of ultra high performance concrete(UHPC), 3 groups of mono-fiber and 10 groups of hybrid-fiber reinforced UHPCs were designed and cubic compression test and four points bending test were performed. Test results show that:for mono-fiber reinforced UHPC, the highest bending stress is 19.98 MPa with a fiber volume content of 5vol% and the bending stress decreases if more fibers are added. For hybrid-fiber reinforced UHPC, bending stress is higher than the mono-fiber reinforced UHPC, the highest bending stress is 23.55 MPa with a fiber volume content of 2vol% and 1vol% for long and short steel fibers, respectively. The flexural response of steel fiber/UHPC is dominantly affected by the volume content of long fibers rather than that of short fibers. The mechanical performance such as bending strength, ductility and flexural toughness are not simply affected by the volume content of long fibers, the balance of volume contents between long and short steel fibers is very critical, the optimal result tested in this experiment is the group of 2vol% and 1vol% for long and short fibers, respectively.