Electrorheology (ER) of heat-treated monodispersed silica particles in silicon oil was investigated using a rotational viscometer. The effect of adsorbed water contained in the silica particles on the ER effect was clarified. The silica particles were prepared by hydrolyzing tetraethylorthosilicate (TEOS). The silica particles were heated at various temperatures and suspended in silicon oil, followed by measuring the ER effect in the silicon oil at room temperature. The heat treatment temperature ranged from 25 to 700-degrees-C. The adsorbed water was characterized by thermogravimetry and IR spectroscopy. The ER effect became maximum at some heat treatment temperature between room temperature and 200-degrees-C. Physically adsorbed water on silica which was removed with heat treatment below 200-degrees-C affected the ER effect. A large quantity of physically adsorbed water, however, decreased the ER effect. The ER effect became negligibly small with heat treatment above 200-degrees-C. A proper quantity of physically adsorbed water was found to be necessary for an appreciable ER effect. The effect of adsorbed water on the ER is discussed.