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The paper contains a mathematical model of the material’s behaviour undercyclic loading taking into account the dynamics of the fatigue process, including thenumber of cycles to failure, induced by the mean stress value. The coefficients in theproposed equation were obtained from experimental tests under symmetrical andnonsymmetrical loading (with the stress ratio R=0). The proposed model was used in orderto modify the energetic criterion with the aim of influence of the mean load on the fatiguelife. The fatigue tests were done for structural steels 10HNAP and 18G2A subjected tocyclic bending and torsion with different values of the mean stress. A good agreementbetween the calculated and experimental results was obtained.