Dynamic viscoelasticity of electrorheological fluids based on microcrystalline c ellulose/castor oil suspensions was experimentally investigated in squeeze flow. The dependence of storage modulus G′and loss modulus G″parallel to external e lectric field on electric fields and strain amplitudes is presented. The experim ents show that, when external electric field is higher than the critical field, the viscoelasticity of the ER fluids converts from linear to nonlinear, and the ER fluids transfer from solid-like state to fluid state with the growth of strai n amplitude. The influences of strain amplitude and oscillatory frequency on the nonlinearity of viscoelasticity were also studied.