Relaxation experiments under simple step-strain shear were performed on MRF-132LD using a rheometer with parallel-plate geometry. The applied step strains vary from 0.01 to 100%, covering both the pre-yield and post-yield regimes. For small step-strain ranges, the stress relaxation modulus G(t, γ) is independent of step strain, where magnetorheological (MR) fluids behave as linear viscoelastic solids. For large step-strain ranges, the stress relaxation modulus decreases gradually with increasing step strain. Moreover, the stress relaxation modulus G(t, γ) was found to obey time-strain factorability. That is, G(t, γ) can be represented as the product of a linear stress relaxation G (t) and a strain-dependent damping function h(γ). The linear stress relaxation modulus is represented as a three-parameter solid viscoelastic model, and the damping function h(γ) has a sigmoidal form with two parameters. The comparison between the experimental results and the model-predicted values indicates that this model can accurately describe the relaxation behavior of MR fluids under step strains.