This paper reports an improved method for modeling and optimization of a MEMS piezoelectric energy harvester (PEH) within desired frequency range with maximum voltage output using finite element method (FEM). Piezoelectric analysis is not trivial due to the difference between the information supplied by manufacturer and the inputs required by FEM tool. A translation procedure using piezoelectric constitutive equations is proposed to address this problem. A parametric MEMS PEH design is optimized with given dimension, natural frequency and stress constraints. Built-in optimization tool of FEM program is used with various optimization methods. Meshing problem of high-aspect ratio structures is also discussed in the paper. Optimization results showed that within 7.5 mm x 4 mm area, 2.92 V can be generated by a PEH with 6.4 mm beam length, 2.2 mm piezoelectric material length.