A highly active electrorheological (ER) material, nano-sized boehmite, AlO(OH), has been obtained. The relative shear stress (r=E/0, 0 and E are the shear stresses at the electric field strength of 0kV/mm and 4.2kV/mm, respectively) of the suspension (25wt.%) in silicone oil reached 212 at a shear rate of 14.5s-1 and a DC electric field. The dielectric property, conductivity, surface energy, microstructure, and composition of boehmite [AlO(OH)] and alkaline alumina containing both AlO(OH) and Al2O3 have been studied. The results showed that the hydroxyl group plays a critical role in influencing the ER activity of the material by changing the dielectric property and conductivity of the material.