Most of the previous studies utilized the maximum applied load and the initial crack length to determine the fracture criterion for Ⅰ-Ⅱ mixed mode fracture of concrete. Actually, the crack has propagated in the ultimate state due to the semi-brittle behaviors of concrete. Therefore, adopting the initial fracture toughness as the fracture criterion seems more reasonable. This subject carried out an experiment of four-point shearing notched beams of concrete with five different heights, i.e, 200mm, 300mm, 400mm, 500mm and 600mm to determine the initial cracking state according to sudden decrease of the strain gauges’ values. Then the initial cracking load ini P can be gained. Besides, the quarter point singular element approach was used to calculate the stress intensity factors ini I K and ini II K corresponding to ini P in the present study. Moreover, different combinations of ini I K and ini II K were obtained according to different positions of pre-cracks. Correspondingly, the Ⅰ-Ⅱmixed mode fracture initial cracking criterion for concrete can be gained using the curves of track formed by the different combinations of ini I K and ini II K . Through analyzing the curves of track under different specimen heights, it was found that the size effect exists significantly in the specimens with heights from 300mm to 500mm, however, the curves in the 500mm and 600mm specimens approach each other.