The mixed mode Ⅰ-Ⅱ crack propagation behaviors when K _Ⅰ=K _Ⅱ were studied based on the geometrical modeling for stress singularity boundary multi-direction cracking, the energy release rate and the energy-based driving force of crack side-branching and kinking had been established, and the critical initiation angle or the critical stress intensity factor had also been calculated. The prediction of critical crack kinking angle by the present theory had a good agreement with 2024-T3, and the deviation was estimated to be 8.9% when compared with En3B and Al.