The material removal is investigated in electrorheological (ER) fluid-assisted polishing of conductive materials. The combination structure of ER particles with abrasive particles in ER polishing fluid is discussed when the electric field is applied. The forces exerted on the abrasive particles for material removal are analyzed. According to Preston’s equation, a model for material removal of the polished conductive workpiece is derived. Polishing experiments with diamond particles for WC is conducted and the experimental results are compared with the theoretical predictions, which confirm the validity of the presented model.