Abstract A new criterion has been developed for the strength of rocks in the condition of true triaxial stress. The criterion parameters are calculated based on the results of a complex determination of the ultimate strength, which includes uniaxial tensile, uniaxial compression and biaxial compression. The results of the complex determination of strength indicators are presented, as well as of the parameters of the proposed criterion for various rock types. The dependences of siltstone strength on the values of intermediate and minimum principal stresses have been formulated. The results of the comparative analysis are given on the theoretical and experimental data obtained by Karman, Becker and Mogi for various load patterns.