Abstract: The mechanisms of concrete fracture process and subcritical crack growth in mesoscale level have been systematically investigated through the model tests and the three-point bending fracture tests with Scanning Electron Microscope (SEM). It is shown that the fracture of concrete behaved complicatedly and irregularly, which underwent explicitly a subcritical crack growth prior to the unstable crack propagation. The subcritical crack growth appeared a kink path rather than a straight path assumed in the conventional fracture mechanics. The subcritical crack growth and the critical unstable growth behaved the fractal features. The initial fracture toughness K_ICⁱ and the fractal equivalent fracture toughness K_IC^e,f were proposed to quantify the resistance capacity against the initial crack propagation and the critical unstable crack propagation in concrete. The formulas of the subcritical crack growth length, the initial fracture toughness K_ICⁱ and the fractal equivalent fracture toughness K_IC^e,f were deduced in this paper. It indicates that the fractal equivalent fracture toughness K_IC^e,f at the critical unstable crack propagation is positively proportional to the fractal dimension of subcritical crack propagation.