This paper presents an optimal design method to maximize the efficiency of a brushless permanent magnet motor with an exterior rotor for blower system. A genetic algorithm is developed to find the optimal variables based on an objective function for the efficiency of the designed motor where the constraints and independent variables are given analytically with an equivalent magnetic circuit. Using the genetic algorithm and the equivalent magnetic circuit, it is possible to achieve a design to satisfy the desired performance requirements within a reasonable geometry. The finite element analysis is then used to confirm the optimality of the design. Computer simulation results are given to show the effectiveness of the proposed design process that can achieve a good prediction of the motor performance.