Ultrasonic guided waves have shown great potential for long range inspections of structures with large areas, such as pipes, vessels, rails, etc. The possibility of finding defects depends on whether scattered waves from a defect are detectable. For example, for a liquid-filled pipeline inspection using torsional waves, the incident waves may be converted to longitudinal waves, which could then leak into the liquid and become undetectable. Therefore, the wave mode conversion study of a defect, especially a 3-dimensional defect, becomes an interesting and valuable topic. The wave scattering and mode conversion of a through-wall hole in a plate was studied both experimentally and theoretically. Mixed mode conversions from SH modes to Lamb modes were demonstrated utilizing electromagnetic acoustic transducer (EMAT) guided wave techniques and a hybrid 3-D boundary element method. This mixed mode study could provide very useful guidance for guided wave inspection methodology. It is also very helpful in algorithm development for defect discrimination and sizing.