Based on the results of the experimental studies and the data given in the literature, the methodology of determining the endurance limits of structural materials during a complex stressed state in a wide range of cyclic loads is proposed. The semi-empirical dependence, which is suggested, makes it possible to calculate the values of ultimate stresses in the safest ratio, and the use of the straight line, which is set by the angle tangent on the plane combined with the Serensen classical criterion to determine the least safe ratio of them. The use of the proposed hyperbolic curve to determine the ultimate stresses increases the values of safe amplitudes for the whole interval of their interaction and reduces the difference between safe and un safe values. Its advantage in dtermining the amplitudes of such stresses, the amount of necessary tests is considerably reduced. And, taking into account the indicated margin of safety (12–14%) of analytical calculations for well-studied structural materials for linear (uniaxial) cyclic stressed state, in some cases, it is possible to avoid their experimental verification. At the same time, a wider range of information about the physical and mechanical properties of the material under study is used. It is established that the difference in the values of amplitude stresses calculated by the proposed method and experimental data in the specified range of jointly acting limit values contributes to the strength reserve and reliability on the endurance limit of real structural elements operating under conditions of repeatedly alternating loads.