The transient process of an electrorheological (ER) fluid based on zeolite and silicone oil sheared between two parallel plates to which a square-wave electric field is applied has been experimentally studied. The transient shear stress response to the strain or time is tested. The characteristic constants of time under different applied electric fields and shear rates have been determined. The response time is found to be proportional to shear rate with an exponent of about -0.75 in the tested shear rate range, which agrees with the theoretical predictions made by others. But it only shows a small dependence on the strength of the applied electric field. The results show that the transient process of ER fluids is related to the structure formation in the shearing. When the required shear strain is reached, the shear stress rises to a stable value under constant electric field. Although the electric field strength greatly affects the yield strength, it shows little effect on the stress response time. Also, experiments showed the electric field-induced shear stress decreased with an increase of shear rate.