This paper presents a concept about activating and optimizing procedures of an activated anti-rolling tank system. The concept consist of utilizing a numerical method of analyzing a feedback control system of ship roll motion in waves by a U-section anti-rolling tank and of utilizing an orthodox analytical method (U-tube theory). The computer program of the numerical method is newly remade for fluid dynamic motion in the tank on the basis of the SOLA-SURF scheme. A series of numerical computation are executed for six anti-rolling tanks by using the newly-remade computer algorithm, and the damping characteristics of fluid tank and the period of natural oscillation of fluid in the tanks are estimated by means of analyzing the numerical results. And, several problems are discussed for activating and optimizing procedures of the fluid tank system for ship stabilization.