Electrorheological response was experimentally studied by the use of silicone oil suspensions containing submicrometer-sized and supermicrometer-sized silica particles with different amounts of adsorbed water, The simultaneous measurements of dielectric permittivity and shear stress of the suspensions were carried out after the application of alternating current voltage under steady shear in ranges of shear rate (150-1400 s(-1)), electric field strength (0-4 kV/mm), its frequency (30-1000 Hz), and particle volume fraction (0.1-0.3). For the particles with small amounts of adsorbed water, steady shear stress was attained within several minutes after the application of electric field, The steady-state data for both the particles at each electric field strength showed that the shear stress generated by the application of electric field, Delta tau, varied correlatively with the generated permittivity, Delta epsilon(r) (= epsilon(r) - epsilon(r, oil)), where epsilon(r) and epsilon(r, oil) are the permittivities of the suspension and the silicon oil, respectively, Under a wide variety of experimental conditions, the steady-state data for both the particle sizes could be correlated with a simple relationship, Delta tau proportional to (Delta epsilon(r)E)(2), where E is electric field strength. For the particles with large amounts of adsorbed water, steady state was not attained, and the evolutions of shear stress and permittivity of the suspensions were measured after the application of electric field. Remarkably, the transient values of Delta tau varied with (Delta epsilon(r)E)(2) and fell along the same correlation line as the steady-state data.