In conjunction with the transfer matrix techniques for modeling ultrasonic wave in multilayered composite medium, the spring model for a weak interface and the interface condition of the slip interface, we have the generalized frequency equ ations for low-order mode guided waves in ultra-thin layered composites structur e with a weak or slip interface in different depth, and the effect of the depth of the interface on the dispersion properties of low-order mode ultrasonic Lamb wave are analyzed. The “ultra-thin” here means that the thickness of the plate is only a fraction of the ultrasound wavelength so that the echoes from the fro nt and the back of the surface cannot be separated in the time domain.