The purpose of this paper is to improve the efficiency of the single -phase PW M DC-AC inverter based on the optimization of the switching-phase of the PW M. The improvement of the efficiency means to improve the quality of the output waveform, the efficiency of the conversion, and the reduction of the switching -loss. W e propose an evaluation function that corresponds to the minimum value of the evaluation function by using the particle swarm optimization algorithm. W e improve the efficiency by setting a criterion of each index. As the result, the proposed algorithm can reduce high harmonic components in the switching times s maller than that of the conventional triangle wave comparison method. Also, we confirm that the algorithm can improve the quality of the output waveform and the efficiency of the conversion.