Abstract: The contour and characteristic sizes of a micro-crack zone ahead of a Fracture Process Zone (FPZ) were derived by the local solution based on the stress functions of Westergaader and the whole solution of the crack tip stress field based on Muskhelishvil results, respectively. Furthermore, the size and effective size of FPZ are solved by the use of a power exponent model considering the effect of strain softening. Meanwhile, the influences of three strength criterions of brittle failure on FPZ sizes are also studied. The discussion show that FPZ contour sizes determined by Moiré criterion and maximum strain criterion increase with the decreasing tensile-compressive strength ratio and Poisson ratio, and approach that of the maximum stress criterion. The size of FPZ deduced by local solution is only suitable for a larger specimen under a lower stress level.