Piezoelectric transformers are expected to be small, thin and highly efficient, and which are attractive as a transformer with high power density for step down voltage. For these reasons, we have attempted to develop a step-down piezoelectric transformer for the miniaturized adaptor. we propose a piezoelectric transformer, operating in thickness extensional vibration mode for step-down voltage. This transformer consists of a multi-layered construction in the thickness direction. In order to develop the step-down piezoelectric transformers of 10 W class and turn ratio of 0.1 with high efficiency and miniaturization, the piezoelectric ceramics and piezoelectric transformer designs are estimated with a variety of characteristics. The basic composition of piezoelectric ceramics consists of ternary yPb(ZrxTi1 − x)O3·(1−y)Pb(Mn1/3Nb1/3Sb1/3)O3. In the piezoelectric characteristics evaluations, at y = 0.95 and x = 0.505, the electromechanical coupling factor(kp) is 58%, piezoelectric strain constant (d33) is 270 pC/N, mechanical quality factor(Qm) is 1520, permittivity (ε 33T/ε0) is 1500, and Curie temperature is 350°C. At y = 0.90 and x = 0.500, kp is 56%, d33 is 250 pC/N, Qm is 1820, ε33T/ε0 is 1120, and Curie temperature is 290°C. It shows the excellent properties at morphotropic phase boundary regions. PZT-PMNS ceramic may be available for high power piezoelectric devices such as piezoelectric transformers. The design of step-down piezoelectric transformers for adaptor proposes a multi-layer structure to overcome some structural defects of conventional piezoelectric transformers. In order to design piezoelectric transformers and analyze their performances, the finite element analysis and equivalent circuit analysis method are applied. The maximum peak of gain G as a first mode for thickness extensional vibration occurs near 0.85 MHz at load resistance of 10 Ω. The peak of second mode at 1.7 MHz is 0.12 and the efficiency is 92%.