Abstract: In this paper, a new type of ultrasonic transducers consisting of the longitudinal sandwich vibrator and the fiexural circular thin plate was studied. The resonance frequency equations were derived, the effective electromechanical coupling coefficient wasmeasured and the radiating characteristics of the compound transducer were analyzed. Experiments show that the compound transducer has many resonance frequencies,when the frepuency of the longitudinal vibrator is equal to that of the fiexural circular thin plate, the vibration of the compound transducer is ideal and the effective electromechanical coupling coefficient is high, therefore, it proves a promising ultrasonic radiator in liquid.