Abstract: 3D scattering of obliquely incident plane SH waves by a cylindrical cavity in a layered half-space is studied using indirect boundary element method. The validity of the approach is verified by comparison with other papers on this subject. Comparison with models used in engineering shows that the 3D scattering problem cannot be simply decomposed into in-plane and out-of-plane 2D problems, and the simple handling may bring about large errors. The effect of obliquely incident angle on the scattering is also studied, and the result shows that obliquely incident angle has a significant effect on 3D scattering.