An electrorheological fluid composed of sulfonated polymer particles in insulating oil is analyzed on nonlinear dynamic viscoelasticity in oscillatory slit flow. The viscoelastic properties are determined as a function of the electric field, the fluid strain amplitude and the oscillation frequency, and changes in the ER microstructure are studied by direct observation performed by an optical microscope. The results showed that the viscoelastic properties of the ER fluid remarkably depend on the electric field strength and on the strain amplitude. However, no dependence on the oscillation frequency was observed in the range of 2.5 to 6.9 Hz. These dependencies are explained based on the formation and deformation of ER particle clusters that span the electrodes under oscillatory slit flow. All the results are discussed in terms of pre-yield, yield and post-yield region.