Abstract: A scattering matrix was employed to present the mode conversion and scattering properties of Lamb waves near material defects. A 3D FE model was used to study the interaction of the S₀ mode Lamb wave with sub-wavelength cracking. A mode separation method for zero order plate guided waves was developed based on their displacement field characteristics, and used to calculate multi-modal scattering matrixes describing the S₀ mode’s interaction with differently-sized cracks. Results show that the scattering field is non-uniform in the circumferential direction, and related to incident angle. Scattering strength relates to crack length; the shorter the crack length, the more obvious the scattering effect. Crack depth determines the energy portion of each mode. With the increase of crack depth, the scattering energy of S₀ and SH₀ increases, and the A₀ mode first increases and then decreases sharply.