A new unified viscoplastic constitutive equation has been developed for interpreting ratchetting experiments. The model is based essentially on a generalized Armstrong-Frederick equation for the kinematic variable; the coefficient of the dynamic recovery term in this equation is a function of both instantaneous and accumulated inelastic strain which is allowed to vary in an appropriate manner in order to reproduce the experimental ratchetting rate. The validity of the model is verified by comparing predictions with experimental results for austenitic stainless steel (17-12 SPH) tubular specimens subjected to cyclic torsional loading under constant tensile stress at 600°C.