Abstract: The radial vibration of a sandwiched radial composite piezoelectric ultrasonic transducer is studied. The transducer consists of a metal inner core, a prestress tube and a ring group of piezoelectric ceramic piles. Based on the electro-mechanical analogy, the equivalent circuit and the resonance frequency equation of the transducer in radial vibration are derived. The relationship between the first and second order resonance and anti-resonance frequency, the effective electro-mechanical coupling coefficient and the geometrical dimensions of the transducer is analyzed. It is illustrated that the first radial resonance frequency of the transducer are decreased with the inner diameter of the inner core and the wall thickness of the outer prestress tube of it are increased. The second radial resonance frequency of the transducer gets extremum value with the increase of the inner diameter of the inner core, but it decreased monotonically with the wall-thickness of the outer prestress tube increased. Besides, the effective electro-mechanical coupling coefficient of the transducer at the second order vibration mode is higher than that of the first order. Especially, it reach maximum when the piezoelectric ceramic component located in the radial displacement node line. By testing and simulating the radial resonance frequencies of some radial sandwiched transducers. It is shown that the theoretical radial resonance frequencies are in a good agreement with the measured and simulated results.