Abstract: Based on the crack propagation criterion for mode I crack, a new model to calculate K_R extension resistance curves that describe the crack propagation of mode I crack in concrete was proposed by utilizing the initial fracture toughness K ini IC as a material parameter. In this model, the crack extension resistance is composed of the inherent anti-cracking performance of concrete and the resistance caused by cohesive force, and is equal to the driving force of crack extension. The FEM was applied to calculate the crack propagation length and the fracture process zone (FPZ) length in both the three-point bending beams and the wedge splitting specimens, and then corresponding K_R curves were calculated and the size-effect was discussed. The results were compared with those by other methods. It is concluded that as long as the elastic modulus E, the uniaxial tensile strength ft and the initial fracture toughness K ini IC of concrete are determined without measuring P-CMOD curve, the K_R curve can be obtained by utilizing FEM.