AbstractConventional Mohr‐Coulomb strength criterion only describing the relationship between normal stress and shear stress on the failure surface at the peak strength is used as a criterion to determine whether the beginning of rock's failure. This study extends the concept of the Mohr‐Coulomb strength criterion by suggesting that the Mohr‐Coulomb yield criterion can express the stress relationship of strain and stress of failure surface under various stress states after plastic deformation occurs. Based on experimental studies, an experiment approach which employs triaxial compression testing data to determine plastic parameters с and φ which vary with the internal variable κ was established, and parameter expression used to assess the accuracy of isotropic modeling and some testing results were given. The Mohr‐Coulomb yield criterion of isotropic hardening or softening, in which stress‐strain curves of whole process is simplified into a four‐line model using initial, peak and remnant parameters of rock, is more applicable than the trilinear model by Toshikazu. The results of this study provides theoretical and experimental basis for engineering and geological numerical simulation, thus being of instructive importance.