Abstract: A topology optimization method based on a phase-field function is presented for the design of a piezoelectric energy harvester. The material distribution of the energy harvester is determined to maximize the working performance. In the proposed formulation, the distribution of both piezoelectric and base materials (Epoxy) are determined by a phase-field function, considered as a design variable. The design objective is to maximize the energy conversion efficiency of the piezoelectric energy harvester at selected frequencies of operation. Numerical techniques for a sensitivity analysis of objective and constraint functions are presented, and the proposed optimization problem is solved with the method of moving asymptotes. Numerical examples are given to demonstrate the applicability of the proposed mathematical model and design method.