The miniaturization of classical electromagnetic transformers raises certain problems such as manufacture both the coils and the magnetic core, increase in magnetic leak and especially electromagnetic pollution of the environment. One interesting solution consists of using a piezoelectric transformer (PT). In this paper a Rosen-type structure of PT has been chosen. the first approach consists of performing an analytical model. This approach is inspired by the technique used in electro-acoustics and allows building an electric equivalent circuit of the vibrating structure. Analytical equations which describe the PT behavior are difficult to solve as a 3D system. Therefore, it is necessary to use Finite Element Analysis (FEA) tools, in order to take into account two or three dimension effects to select the geometry for its design. Then, with the aim of optimizing the PT performances, a sensitivity analysis using design of experiments and then a FE optimization are carried out. The finite element simulations show that it is possible to increase the transformation ratio by 20%.