It is aimed in this research that a new approach is represented for justifying the influence of specimen size on the mixed mode I/II fracture behavior of rock. The new approach, namely, the MTS-FEM criterion, is an extension of the maximum tangential stress (MTS) criterion in which the finite element method (FEM) is employed for characterizing the tangential stress at the critical distance. Also, two points exist in the MTS-FEM approach for the critical distance. First one is that the critical distance is obtained directly from finite element analysis corresponding to results of mode I fracture test. The second point is that the critical distance from the crack tip is considered as a function of specimen size. The experimental results carried out in the previous studies on cracked Brazilian disks (CBD) with four different radii and under mixed mode loading are utilized with the aim to evaluate the new size effect criterion. It is illustrated that the MTS-FEM criterion is able to predict the mixed mode fracture resistance of rock considering the size effect well without calculating any parameters related to the stress or strain series expansion.
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