In an electro-rheological composition comprising an electrical insulating medium and solid particles dispersed therein, insulating solid particles possessed of morphological anisotropy are used as the solid particles. In a preferred embodiment, the insulating solid particles mentioned above are plate-like insulating solid particles, preferably plate-like insulating solid particles having a diameter (particle diameter) not less than 1 m, more preferably plate-like aluminum oxide particles having a diameter not less than 1 m. In another preferred embodiment, the insulating solid particles which have undergone a surface treatment with organic molecules or a semiconducting inorganic material, particularly the insulating solid particles having a metal oxide such as tin oxide and titanium oxide adhered to the surfaces thereof are used as the particles. Still another preferred embodiment is the ER composition of which electrical insulating medium is gelled.