Abstract: The radial vibration of new radial composite piezoelectric transducers is studied. The radial composite ultrasonic transducer consists of a solid piezoelectric ceramic thin disk and a metal thin ring in radial vibration. The radial vibrations of a piezoelectric thin disk and a metal thin ring are analyzed and their radial electro-mechanical equivalent circuits are obtained. Based on the electro-mechanical equivalent circuits and the boundary conditions between the piezoelectric ceramic thin disk and the metal thin ring, the electro-mechanical equivalent circuit of the radial composite piezoelectric transducer is derived and the resonance frequency equation is obtained. The relationship between the resonance frequency, the effective electro-mechanical coupling coefficient and the geometrical dimensions is analyzed. It is illustrated that the theoretical radial resonance frequencies are in a good agreement with the measured results, and they are also in a good agreement with the results from the numerical method.