To describe the creep rupture process under the complex stress, various equivalent stresses have been considered. From them, the equivalent stress has been selected, at which the total error of the difference between the experimental and theoretical values takes the smallest value among the considered equivalent stresses. Three basic equivalent stresses have been considered, as well as two complex equivalent stresses, which are a linear combination of two basic ones with one material parameter. The analysis of the total errors in the considered experimental data has showed that with the simultaneous effect of internal pressure and axial force on the wall of tubular specimen (or biaxial tension of a plane element), an complex equivalent stress should be used in the form of a combination of the maximum normal stress and the Mises stress. At simultaneous torsion and tension of tubular specimen (or simultaneous tension and compression of a plane element), an complex equivalent stress should be used in the form of a combination of the maximum normal stress and the doubled maximum tangential stress.