Abstract: As shear force is applied to the specimen with a crack,the crack extends and deviates from original crack plane. Measured and calculated shear fracture toughness,KIIc is less than KIc. Theoretical study shows that the crack extension under shear loading is caused by tensile stress,and it does not belong to shear fracture. To restrain or eliminate tensile stress,compressive stress should be applied to the specimen. Thus pure shear fracture caused by shear stress is obtained. Calculated shear fracture toughness KIIc is larger than KIc. The study shows that the occurrence of mode II fracture requires its prerequisite. A prerequisite for mode II fracture is frθmax / fθmax>1 and frθmax / fθmax>KIIc / KIc,where frθmax is the maximum dimensionless shear stress intensity factor,fθmax is the maximum dimensionless circumferential tensile stress intensity factor.