Abstract: Most previous numerical studies merely focused on Internal Solitary Wave (ISW) forces acting on a circular cylinder, and only a few studies have been conducted on ISW forces acting on a square cylinder. A 3-D numerical wave flume is used to investigate the generation and propagation of ISWs. The ISW forces acting on circular and square cylinders are examined through a Large-Eddy Simulation (LES) approach. ISW force behaviors in the circular and square cylinders are studied at various ISW amplitudes. Results show that the forces acting on the circular and square cylinders increase with increasing amplitudes. At the same amplitude, the ISW forces exerted on the square cylinder are larger than those exerted on the circular cylinder because of the more uneven pressure distribution and larger differential pressure between the front and rear of the square cylinder.