The armature reaction of brushless DC motors have important influence on magnetic field distribution, torque ripple, motor inductance and losses. The control mode of surface-mounted brushless DC motors make that the magnetic field circumrotates uniformly but jump every 60deg, and the different angles between stator and rotor magnetic potentials lead to the complexity of armature reaction. The armature reaction under different angles between stator magnetic potential and rotor magnetic potential were computed with finite element method. The computed results provide a precise data base for designing high performance brushless DC motors.