Ultra High Performance Concrete (UHPC) generally uses fibers as reinforcement. Most researches have focused on crack resistance effect of fibers, but the formation and propagation of cracks initiated by fiber-matrix interface are seldom considered. In this paper, three types of fiber orientation directions were designed to study the dual effects of fibers on fracture property of UHPC, and two different kinds of steel fibers have been added into three UHPC to study the influence on interfacial property and fracture property. The results indicated that fibers showed dual effects on fracture property. Fracture energy of UHPC varied from -59.4% to 119.7% when one single steel fiber with different orientation directions was added. The weak fiber-matrix interfacial zone caused the dual effects of fibers. The key to utilize dual effects of fibers is to improve fiber-matrix interfacial property.