Micro cracking in a solid material is a phenomenon of solid plane bifurcation with incompatible microrotation. Therefore analysis of a microrotation field in a strained material is important in fracture mechanics. Based on the strain-rotation (S-R) decomposition theorem newly developed in non-linear mechanics of finite deformation, a new geometric criterion of cracking: critical value of gradient of local mean rotation at a point i.e. |∇ϑ| is proposed. At the same time, the concept of microrotation toughness is established. The advantage and practical value of the new criterion are elucidated through experimental exemplification in the displacement field within 0.2 mm domain around a crack tip. Microrotation toughness values of marble, structural steel and bronze are estimated also.