Abstract:Strength theory of rock is important in safety evaluation and designs of the civil,mining and hydraulic rock engineering,and has important significance in the study of rock mechanics. In this paper,the tri-parameter strength theory under complex stress states, called twin ,is derived. At first,the relations among the compressive strength,tensile strength and shear strength are proposed. Secondly,the brittle failure of rock is analyzed by using the developed twin strength theory and compared with the available experimental data for some rock materials. The results are shown to be satisfactory.