Abstract: Ultra High Toughness Cementitious Composites (UHTCC) can be used as reinforcement material because of its good performances on crack control, energy consumption, corrosion resistance etc. The fracture process of concrete three-point bending beams retrofitted with UHTCC on tension face was studied. Based on the assumption that the interface deformation is consistent and by neglecting the elastic deformation of uncracked regions of UHTCC, a method for determining the total crack width wu of UHTCC is proposed and a simplified model of -wu is obtained subsequently. According to the simplified model, stress value of UHTCC can be determined. Through the analysis of whole process of crack propagation of concrete three-point bending notched beams retrofitted with UHTCC, the initial fracture toughness and the unstable toughness applicable for reinforced concrete were introduced. The double-K fracture criterion is used for describing the whole process of crack propagation of concrete three-point bending beams retrofitted with UHTCC, and then the applicability of the criterion is verified by tests.