The paper describes the development of high performance underwater ultrasonic transducers employing ZnO/Pyrex-glass composite diaphragm structure on a Si substrate. It is shown that the transducer successfully radiates and detects ultrasonics at the fundamental frequency of 155 MHz with a relative -3 dB band width of 11.1% and a minimum transduction loss of 4.3 dB, both of which are in good agreement with theoretical prediction. The possibility of the application of the transducer to sensor devices is described for the measurement of ultrasonic wave velocity in liquid.