An R-curve formula for ultra high performance cementitious composites is derived in this paper. The fracture mechanics based on R-curve is used to predict the load-deflection relation of ultra high performance cementitious composites. The reductions of stress intensity factor and CTOD by steel fiber reinforcement are assumed as conforming linear distribution along crack propagation. The effective numbers of steel fiber on unit area based uniform distribution is used here. Results of the theoretical predictions show a good agreement with test results of three point bending beam of UHPCC. The modified R-curve formula for UHPCC can be a reference for future study of fracture performances of UHPCC.