Current research includes applying new functional materials to mechatronic devices and Electro-Rheological (ER) gel is one of these new functional materials. ER gel enables the development of mechatronic devices which have both high speed response and high levels of safety. In particular, the requirements for power assist devices are high speed response, high safety, and backdrivability. The purpose of this study is to develop an ER gel belt transmission system for use in power assist systems. We applied an ER gel in the power transmission role of a belt transmission system. When the ER gel of this system is de-energized, the input torque is not transferred because of minimal shear stress. When the ER gel is energized by an electric field, the increased shear stress results in a torque transfer. The ER gel belt transmission system functions as a clutch with controlling torque based on these characteristics of the ER gel. The paper proposes such an ER gel belt transmission system, describes it's design and operational features, and presents the results of basic performance experiments.