The shear stresses of ER (Electro-rheological) suspensions are measured by changing a shear rate and electric fi elds (DC and AC) using rotational viscometer whose gap between electrodes can be changed. We use two ER suspensions which are called ERFC and ERFP. ERFC consist of cellulose suspended in silicone oil. The size of cellulose particles is typically about 10-20 μm and a viscosity of silicone oil is 10mm2/s. Cellulose concentration is 20wt.% and the percentage of moisture content is 0.6%. ERFP contains highly sulfonated poly (styrene-codivinylbenzene) particles. An average particle diameter of ERFP is 9 microns and the water content is 2.0 wt.%. Bipolar sine-wave electric fields (AC) are used and the shear rates of 400 s-1 are employed. Amplitudes are between 1.0 and 2.5 kV/mm. The frequency of the electric fields is changed from 0.1 to 100 Hz. Dependence of the shear stress on electrical excitation frequency is investigated.