Abstract: Combining frequency domain retrieve function with low frequency ultrasound, a method is presented in this paper for characterizing ultra-thin plate. By 'ultra-thin', we mean the thickness of specimen is much less than wavelength. The through-transmission retrieve function is examined in detail. In conjunction with the method of least squares) the amplitude spectrum, phase spectrum and complex spectrum are all being used to determine the best estimation, respectively. An analysis of some typical sources of the estimate deviation is presented. The sensitivity of the retrieve function to acoustical parameters and its significance in the propagation of errors have been carried out and well discussed.In experimental study the technique is employed to characterize the brass sheets ranging from 25.4 μm to 152.4 μm with a broadband 2 MHz center frequency transducer. It is observed that the agreementbetween the nominal value and the estimation is excellent.