Electrorheological (ER) effect of dispersions of anhydrous spherical particles with conductive polymer core and nonconductive thin skin formed by chemical treatment was investigated in dielectric fluids. The ER effect was also studied to particles of aluminum for comparison. It was found that shear stress enhancement of both samples depended on what types of methods were used for observation of the ER effect. The ER effect of these dispersions was stronger under ac than dc electric fields in the rotary method, but stronger under dc than ac fields in the vibration method. This is in clear distinction from the behavior of traditional ER fluids containing hydrous particles, in which the ER effect is substantially the same under either ac or dc fields in both methods, and is tentatively related to the shear employed in the measurements.