The paper is focused on the research of the automotive magneto-rheological brake system whose braking force comes from the shear stress of magneto-rheological fluid under the condition of magnetic field. The MRF brake is designed for an electric passenger car to replace a conventional hydraulic disc-type brake. The braking torque of this system can be linearly adjusted by the current in just a few milliseconds with proper materials. Therefore this system has a quick response and precise control performance with a low hysteresis.Nowadays, most of the related research of MRF is about the construction of the prototype and the realization of the brake force. Main limitation of MRF brake lies in the braking torque cannot meet the actual needs and the power consumption may be too much if it is not well designed. The prototype introduced in the SAE Brake Colloquium-31nd Annual has been manufactured and assembled critically. Some necessary experiments that can show the performance of MRF have also been done to get some essential data. Based on the data of bench test, the improvement measures are proposed to improve the magnetic induction intensity in the working space and the brake torque.Then the research is focused on optimal design using the method of finite element integrated with optimization tools. In the optimization, the required braking torque, high shear speed, the saturation of magnetic field and all significant geometric dimensions are considered. The other important part of the design is about the layout of MRF coils. Through comparing different distribution of magnetic field, coils can be arranged effectively according to the different structures. According to the results, effective MR brake for the passenger car may be obtained and some discussions on the performance improvement of the MR brake are described. Besides, some useful design experience is acquired and optimal results can also be seen in this paper.