In this paper, a new method based on the evolutionary algorithms is proposed for optimal design of a slotless permanent magnet brushless DC (BLDC) motor with surface mounted magnets. BLDC motor is the ideal choice for applications that require high reliability, high efficiency, and high power-to-volume ratio. These motors are widely used in sensitive applications such as medical instruments, space crafts and submarines. In these applications having high efficiency, minimum volume and cost are very important. So, in this paper, the objective function of design is presented considering efficiency, volume and cost, simultaneously. The modified social spider algorithm (MSSA) is one of the newest evolutionary algorithms whose capabilities have been proved in solving complicated optimization problems. The MSSO algorithm must find the optimal value of geometric parameter of motor including number of pole pairs, cross sectional area of wire, pole–arc per pole–pitch ratio, magnet thickness, stator/rotor core thickness, winding thickness, mechanical air gap, rotor radius, current density, wire gauge and stator/rotor axial length to minimize the power losses, volume and cost. The simulation results show that the proposed method has excellent performance in slotless permanent magnet BLDC design.