In this study, Underwater explosion was simulated with the High Speed Craft (HSC) to observe the behavior and structural response of the HSC. Finite element model was made by True Grid and the simulated with LS-DYNA. As the floating ship in water was a kind of fluid and structure interaction (FSI) problem, the Arbitrary Lagrangian/Eulerian (ALE) method was used. This thesis conducted simulation by modeling HSC with UNDEX. The DTMB 4667-1 Model was used as HSC model which was mono-hull and 100kg TNT was modeled as explosive charge with water and air. The result showed the ship behavior and structure response for several different cases of underwater explosion in different distance and different direction from HSC when it was running in high speed.