Electrotheological (ER) fluid has notable characteristics of viscosity. Viscosity of ER fluid can be varied over a wide range in about several milliseconds according to the applied electric field. In this paper, an active small rotary damper using ER fluid is made on an experimental basis and its applications to velocity control and impedance control are examined. The characteristics of the rotary damper are small, simple structure and fixed electrode type. Then, it is suitable for control experiments because it can be incorporated into mechanical system safety and easily. As ER fluid is fairly nonlinear and the control of the ER damper is complicated, fuzzy set theory is applied. Responses of the angular velocity and impedance characteristics of the ER damper are desirable in experimental studies.