In a spherical piezoceramic transducer for HIFU (High Intensity FocusedUltrasound) treatment, a mode of plate wave propagation within the ceramic wereexperimentally observed in addition to the pure thickness-mode vibration. Inthis study, we experimentally and numerically analyzed the vibration behavior ofan air-backed single-element spherical PZT transducer. The acoustic pressuredistribution on the ceramic surface was obtained in the experimental analysisusing time reversal, and the numerical analysis was performed using FEM. In thetwo-dimensional Fourier analysis, the radial wavenumber spectrum showed asecondary peak, corresponding to a leaky Lamb-like mode. The leaked waves mayhave propagated into water and formed the relatively large lobes on thepropagation axis. These unwanted nearside lobes may potentially cause additionalnear-field heating of the tissues which should not be treated.