This paper provides an investigation of torque transmission system using ERF (Electro-Rheological Fluid). The torque transmission system using ERF is a new conception device because an apparent viscosity of ERF can be changed by applying an electric field, We use the coaxial cylinder type in which the ERF fills the annular space between a pair of coaxial cylindrical electrodes and experiment results show that the measured torque was increased with the increase of the electric field. These are analyzed to provide guidelines to assist in the development of practical ER devices.