The Hydrofoil Catamaran (HC) concept has been developed based on the tank test at University of Tokyo. In 1990, the prototype for HC named “Exceller” was built and actual sea testing was conducted. The Exceller has flaps and ailerons for pitch, heave and roll control. This paper describes the Roll Stability and Control Augmentation System (Roll SCAS) by using ailerons and its validation at sea. The NEC-PC 32 bit computer was installed on the ship to form the data aquisition and the digital control system. The dynamics of actuators and the ship were obtained by the measurement of the actual hardware. Roll angle and roll rate were used as feedback variables with gains, and gains were tuned by the comment of seamen. The Roll SCAS was exemplified at actual sea and showed clear distinction between its on and off.