Abstract: An accurate mathematical model of radial suspension force in a bearingless brushless DC motor is of great significance to reduce the complexity of system and to improve the control accuracy.This paper began with the structure of the bearingless brushless DC motor,then elaborated on the working principle and established the equivalent circuit.The magnetomotive force of permanent magnets was equivalent to virtual currents in suspension windings,and then a mathematical model with single degree of freedoms for radial suspension force was derived.Finally,results by finite element analysis and theoretical values were compared.In order to get more convincing results,the correction factor was introduced for the displacement stiffness coefficient,and then the finite element analysis was carried out again.The ultimate results show that the proposed mathematical model of radial suspension force of the bearingless brushless DC motor has feasibility and high accuracy.