Abstract: The shear stress of suspensions based on zeolite and silicone oil under different dc elelctric field and different particle volume fractions was investigated. Results showed that the suspensions behaved as Newtonian fluid in the absence of electric field and as Bingham fluid in the presence of an electric field. The shear yield stress of the suspensions increased with the increase of the applied field strength. It increased greatly with the increase of particle volume fraction as well. The relationship between the volume faction and the shear yield stress is even higher than that predicted by the inverse ratio between the particle interaction force and the square of gap between he particles. This was attributed to the distortion of the adsorbed additives that might decrease the real distance between the particles and the multi-body effect of ER fluids under high volume fractions that significantly increased the shear yield stress.