A 48 V belt driven starter generator (BSG) motor controller for mild hybrid vehicle is developed. The overall principle diagram of electrical design is described. The structure and hardware scheme of the 48 V BSG motor controller are analyzed in detail. A design scheme of 48 V BSG motor controller based on the MOSFET module is proposed. The thermal simulation of the heat dissipation baseplate is carried out to study its heat dissipation effect on MOSFET module. Finally, the bench test of the 48 V BSG motor controller prototype is carried out. The test results show that the 48 V BSG motor controller has a good control performance.