A Zeolite-based ER fluid was sheared in a cup-bob unit under stepwise electric field excitation. The shear stress of the ER fluid, electric current and the applied electric potential during the transient phase were recorded simultaneously with a data-acquisition system. The experiment results show that there exists a delay time of approximately 0.3ms in shear stress following the change in electrical potential. The delay time corresponded to the time needed for the charging and the discharging of the ER fluid cell at the moments of field-on and field-off respectively, and was not influenced by the field strength and the shear rate. This result implies that the shear resistance of ER fluid does not change in the polarization and de-polarization processes. With the increase in shear rate, transient periods of stress build up and decay occurred at the switching on and switching off moments following an exponential pattern. The response time of ER fluid is discussed in the context of the designing of ER devices.