The study of stress state under shear load indicated that shear load applied to crack induces tensile fracture instead of shear fracture. It leads to consider that loading mode does not always correspond to fracture mode. Under different mixed loading, fracture belongs to single tensile fracture in most cases, only in certain special case shear fracture can occurs. Thus fracture mode should be distinguished from loading mode. To judge what fracture mode will occur under arbitrary loading prerequisites for mode I and mode II fracture become vital important. The criterion to define mode I or mode II fracture under any plane loading can be summarized as: prerequisites for mode I fracture are: 1), fθmax>frθmax or frθmax/fθmax<1, or 2), frθmax/fθmax>1, but frθmax/fθmax<KIIC/KIC; prerequisite for mode II fracture is frθmax/fθmax>1 and frθmax/fθmax>KIIC/KIC, where fθmax is the dimensionless maximum circumferential stress intensity factor and frθmax is the dimensionless maximum shear stress intensity factor.