Iwan models are used to simulate the non-linear and hysteretic behaviour of soils under cyclic loading conditions. However, the model in its original form cannot take into account the stiffness degradation which is observed during cyclic loading of soft clays. Studies show that the stiffness reduction (expressed as degradation index) can be represented as a function of the number of cycles and of a degradation parameter depending on the strain amplitude in the case of strain controlled cyclic loading tests. This degradation index has been incorporated into Iwan's series-parallel model as a single fatigue parameter to account for the degradation during cyclic loading. The comparisons made with the existing results of two marine clays tested under undrained cyclic triaxial and simple shear conditions provided an opportunity to understand the capability of the one dimensional model.Iwan models are used to simulate the non-linear and hysteric behaviour of soils under cyclic loading conditions. However, the model in its original form cannot take into account the stiffness degradation which is observed during cyclic loading of soft clays. Studies show that the stiffness reduction (expressed as degradation index) can be represented as a function of the number of cycles and of degradation parameter depending on the strain amplitude in the case of strain controlled cyclic loading tests. This degradation index has been incorporated into Iwan's series-parallel model as a single fatigue parameter to account for the degradation during cyclic loading. The comparisons made with the existing results of two marine clays tested under undrained cyclic triaxial and simple shear conditions provided an opportunity to understand the capability of the one dimensional model.