Micro cracking in solid material is a phenomenon of solid plane bifurcation with incompatible micro-rotation. Therefore it is important to analysis the micro-rotation field in a strained material in fractute mechanics. On the basis of strain-rotation (S-R) decomposition theorem, the geometric criterion of cracking has been established. In order to verify its validity, using grating experimental method the thin compact tension specimens were investigated. The analysis results show a good agreement between the experimental and geometric criterion.