In this paper, in order to enhance thrust of slotless type Permanent Magnet Linear Synchronous Motor, an optimal design is achieved by combining a genetic algorithm with 3D space harmonic method. In the case of multi-objective functions, the ratio of thrust/weight and thrust/volume are increased by $7.56[%]l;and;7.98[%]$, respectively. Thus, miniaturization and lightweight were realized at the same time.