This paper presents a numerical method of analyzing a feedback control system of rolling motion of a ship in waves by a U -section type activated antirolling tank. The antirolling tank system is activated by impellers installed on the channel's centre of tank. The presented method consists of a simulation technique of liquid motion in the tank with impellers and of a design procedure of a feed back control system. The simulation technique is remade for the liquid motion in a U -section tank on the basis of the Marker-and-Cell method and its computer program (sloshing cord). The design procedure is based upon a theory of control system (the Root locus method). A series of numerical computation are executed for three activated antirolling tanks by using proposed method in any conditions, and several problems for analyzing a feedback control system are discussed.