Abstract: Cyclic loading tests with variable strain amplitudes were conducted on 10Ni5 Cr Mo V steel with nominal yield strength of 785 MPa and cyclic stress-strain behavior was analyzed.Resultsindicate that elastic modulus and yield stress both decreases linearly with increasing strain amplitude. Linear relationship was found between true stress and true strain logarithm in plastic deformation region. A single logarithm constitutive model for cyclic deformation σ=σ_p+χ·lgε_p was proposed based on above results and the parameters of this model for 10Ni5 Cr Mo V steel were determined.