Abstract: In practical projects, structural fracture often undergoes complex stresses. The studies on the fracture mechanism of mixed mode cracks are of important theoretical significance and practical implication. Based on the mixed I - II crack mode and the linear elastic theory, the geometrical characteristics and stress form of cracks are considered, and the theoretical solution of the stress intensity factor (SIF) at the crack tip is introduced. The radial shear stress criterion and the twin shear stress criterion for mode II fracture are put forward. For the I - II mixed-mode open crack, a new method to determine the fracture type is proposed by comparing the SIF ratio of equivalent mode I and II with that of mode I and II fracture toughness. In addition, an appropriate fracture criterion for mode I or II fracture is used to calculate the theoretical initiation angle. The compression-shear fracture tests on the brittle rock-like samples with a single open pre-crack are carried out under compression. The results indicate that the theoretical initiation angles and the experimental results are in good agreement.