Abstract: As far as concrete is concerned, before the unstable propagation of crack, nonlinear deformation has been produced because of microcracks at crack tip and accompanied with prepropagation of crack. The nonlinear deformation and prepropagation make concrete absorb much more energy before fracture. In this paper, the nonlinear fracture toughness G_Ic which resists the crack propagation is obtained from the nonlinear load-displacement curve in experiments. The influences of notched length to specimen height ratio and the specimen height on G_Ic are investigated.