The frequency dependence of the shear stress of metal‐particle/insulating oil electrorheological (ER) fluids under an ac electric field has been studied. The results show that the shear stress of the metal‐particle ER fluid under a fixed electric field increases with increasing frequency and then tends to a saturated value at higher frequency. The shear stress at 103Hz is much larger than that under a dc field. The transition of the shear stress shifts to higher frequency when the conductivity of the fluid is larger. The measurements are consistent with the theoretical model proposed by Davis [J. Appl. Phys.73, 680 (1993)]. ©1995 American Institute of Physics.