Abstract：Unified strength theory is the development of Coulomb criterion with analyzing the effect of intermediate principal stress. It deems that rock has the same strength under conventional triaxial compression and tension；and a zigzag is used to describe the effect of intermediate principal stress on strength. The theory has been studied deeply and used widely. The true triaxial strengths of eight rock samples using the theory are fitted；and the processes of parameters determination and misfits of the fitting solutions are also given. If the parameters of Q and K are determined from the relationship between pseudo-triaxial strength and confining pressure，and only parameter b is determined from true triaxial strengths，the theory can not describe main characteristics of the influence of intermediate principal stress on strength. The misfits for several rock samples may be higher than those from Coulomb criterion. If the parameters of Q，K and b are determined on the least misfit to the whole set of strengths，the theory has a pretty fitness. However，the pseudo-triaxial strengths of several rocks from the best fitting solutions have great scatterness from the real magnitudes. Also，it does not correspond to the experimental results that rock has the same strength under conventional triaxial compression and tension；the maximum increase of strength from intermediate principal stress has a linear relation with the minor principal stress；and the first-order derivative of strength has an interruption at the peak point. Insufficiency of experimental results and dispersion within test data make great difficulty in reviewing the feasibility of the best fitting solutions.
