According to the decomposition of principal stress into hydrostatic stress and deviatoric one as well as to the correlation between them in Mohr's circles and deformations, the paper deduces the mathematical expression of modified Mohr-Coulomb's theory. This expression solves many shortcomings of Coulomb's theory and Mohr's theory theoretically and particularly, it describes the influence of intermediate principal stress on shear strength. At the same time, a four-parametric criterion is established. The computing results using this four-parametric criterion are quite agreement with those obtained from tests on concrete specimens of different kinds.
