Stepwise compression of electrorheological (ER) fluids based on zeolite and silicone oil under constant voltages was experimentally investigated. The difference between peak compressive stress and stable compressive stress obtained during the stepwise compression changed with the change of applied voltages and compressive strains. The decay ratio of compressive stress, which may depict the solidification level of ER fluids under external electric fields, decreased with the increase of electric field and compressive strains. Also both the peak compressive stress and the stable compressive stress were shown to be determined by the electric field under low applied voltages. With the increase of the applied voltage, ER fluids were shown to be described by the mechanics of compressing a continuous fluid. At a much stronger ER effect, a deviation from the prediction of the continuous media theory occurred, and structure strengthening of ER fluids by compression should be considered