A ferroelectric TGS particle/silicone oil electrorheological (ER) fluid is introduced to investigate the dielectric dependence of the ER effect. The dielectric constant of ferroelectric changes violently with temperature at the Curie temperature (Tc). By measuring temperature dependence of shear stress across Tc, the effect of dielectric constant on shear stress can be directly obtained. All the results are more reliable due to the same conditions, such as size, shape, composition of particles, as well as the same chemical nature of particles and interface property between particles and liquid. The measurement was carried out under a high-frequency (1000 Hz) ac electric field where ER effect is dielectric constant dominated. For the first time, the dependence of the ER effect on dielectric mismatch has been quantitatively obtained experimentally. There is an obvious deviation of available theoretical calculations from our measured data. A more rigorous theoretical study should be developed to quantitatively interpret the relation of the shear stress and the permittivity mismatch factor. © 1998 American Institute of Physics.