Abstract: The radial vibration of a composite ultrasonic vibration system is studied.The system is com-posed of a piezoelectric ceramic thin ring that is polarized in its thickness direction and a metal thin circular ring.The radial vibrations of a piezoelectric ceramic thin ring and a metal thin circular ring are analyzed respectively,and the corresponding radial electro-mechanical equivalent circuits are obtained.Based on the equivalent circuits and using the boundary conditions,the electro-mechanical equivalent cir-cuit of the radial composite ultrasonic system is derived and the resonance frequency equation is obtai-ned.The relations between the resonance frequency,the anti-resonance frequency,the effective electro-mechanical coupling coefficient and the radius ratio are analyzed.It is illustrated that with the radius ratio increasing the resonance and anti-resonance frequency increase and the effective electro-mechanical coupling coefficient for the first radial vibration decreases.Experiments show that the measured resonance frequency and anti-resonance frequency are in a good agreement with the theoretical results.