Abstract: The unified solutions for the shape and size of mode I crack tip plastic zone are obtained based on the twin shear unified strength theory.The strength difference effect(SD Effect)and the intermediate principal stress effect are all included in the solutions.Former solutions for this problem using Tresca criterion,von Mises criterion,and Mohr-Coulomb criterion are special cases or linear approximations of this solution.The shape and radius of crack tip plastic zone of type I cracks for concrete structure is given in this paper.It can be found that tension-compression strength ratio has obvious effect on the plastic zone of mode I crack tips.The influence of the parameter b,which reflects the intermediate principal stress,has stronger influence on the plastic zone as the tension-compression strength ratio is larger and vice verse.This solution is applicable to various types of materials and of important practical value.