Most of the ultrasonic surgical or therapeutic devices apply the outside body designs with only the vibration tip contacting and delivering energy to the tissue. For inside body applications, a miniaturized 421 kHz piezoelectric transducer with OD 2 mm and length 10.35 mm based on longitudinal vibration mode was developed in this paper. The transducer is a one-wavelength design with a stepped horn to focus the energy. The theoretical analyses on mechanical, electrical and acoustic properties were performed using finite element analysis (FEA) and equivalent circuit (EC) model, which showed consistent results. The Qm and keff were found to be 393 and 0.21, respectively. The maximum acoustic output power was observed to be 27 mW at 10 V input when the depth of immersion is 1/4 wavelength. Ultrasonic streaming was also observed when the whole transducer was immersed in the water and the input power level was in the order of 0.6 W.