ER fluid is a unique functional fluid that changes its rheology when an electric field is applied. The property of an actuator using ER fluid is different from conventional actuators such as DC or AC servomotors. In this study, we developed a new actuator using ER particle-type fluid called an ER actuator. The developed actuator can produce two directional torques. We derived simple. models of the ER actuator from reference torque to output torque and velocity characteristics. Then we verified the validity of the models and control performance of the actuator by conducting the velocity and position control experiments.