Abstract: Based on the fictitious crack model and Critical crack tip opening displacement CTODc, the precritical crack propagation length △a_c. of concrete was determined by means of three point bending beams. According to the calculation value of △a_c, the effective fracture toughness K_Ic^un of concrete was evaluated. It is shown that △a_c increases with the increase of the concrete specimen size, but K_Icⁱⁿⁱ and K_Ic^un are size-independent and can be considered as fracture parameters of concrete. It is also found that the linear elastic fracture toughness criteria are applicable to the crack evaluation of concrete structures.