Regenerative braking is an advanced braking system that is used in conjunction with traditional braking systems, converting the kinetic energy of the braking process into electricity for reuse. With the research and design of regenerative brakes using magnetorheological fluid (MRF), the rotor shaft of the generator will be driven through the active shaft thanks to the power transmission properties of MRF during operation. When not in operation, the brake mechanism is not supplied with current, the oil layer is in a liquid state, the active shaft is not linked to the fixed shaft. When braking, the system will supply an electric current, and the MR fluid layer changes to a solid state to drive from the active shaft, linked to the fixed shaft, rotating the generator rotor. With the application of the regenerative braking using magnetorheological fluid, it is possible to optimize the power source on rear-wheel drive passenger cars. In this research, a new shape of regenerative brakes using magnetorheological fluid is designed. Using magnetic field simulation and analyzing calculated data to evaluate the efficiency of the field current and the generated voltage.