Changes in the surface state of gibbsite (GB) by milling were elucidated by the electrorheological properties of a suspension comprising GB and silicone oil. Milling GB brought about increase in the amount of readsorbed water as a consequence of mechanochemical dehydration with simultaneous amorphization. In order to discuss the effects of structural imperfections of the substrate induced by milling on the degree of surface polarization, the amount of adsorbed water was precisely controlled by heating at 200degreesC and subsequent exposure to an atmosphere with predetermined humidity. Compared at the same amount of adsorbed water, the yield stress under 2 kV/mm of the suspension with GB milled for 2 h was about 5 times larger than that of GB milled for 0.5 h. A similar tendency was observed from the creep behavior of the suspension. These electrorheological properties are discussed in the light of enhanced polarization of surface OH groups on the mechanically activated surface of the substrate.