An Attitude Control System for a high speed catamaran with contorl fins in waves is designed and investigated by the experimental methods. An optimal control theory is applied to design the automatic attitude control system of the high speed catamaran for its good seaworthiness. The experimental results performed in KRISO model basin for the displacements and accelerations for the model ship are measured in regular hear waves. Form the epxerimental results the effect of control planes is thought to be incroeasing with speed.