The paper examines the weakly nonlinear behavior of a 5-layer beam-like piezoelectric transformer operating near resonance, where the main structure of the device consists of properly poled and electroded flexible piezoceramic four-layers separated by a central metallic layer. Nonlinear effects of the large deflection to induce the incidental in-plane extension near resonance are considered, which is shown that on one side of the resonant frequency the output-input relation becomes nonlinear, and the other side output voltage experiences jumps.