Abstract: Strength theory for complex rocks and soils under the condition of plane strain is given. Three dimensional failure criterion of nonhomogeneous nonlinear anisotropic rocks and soils is used. The associated rules of flow are derived from the criterion. Plain strength criteria are established. These criteria are generalization of the Mohr-Coulomb criteria in anisotropic materials. Many classical criteria may be obtained through simplifying the criteria proposed. The strength theory presented is significant to engineering design in complex rocks and soils.