A method for quantitative evaluation of the effect of seakeeping performance on ship's mission is presented. A concept of mission-effectiveness is introduced which is used as a quantitative index of the effect of seakeeping performance for ship system capability. In order to evaluate the mission-effectiveness, a formulation, commonly used in the reliability engineering, is applied, considering that the ship system is presented by the sum of subsystems onboard, grouped under their functions. For the determination of the effectiveness of subsystems, performance degradation of subsystems due to ship responses are made and/or assumed by using results of research works about seakeeping quality of ships so far. The present method is applied to evaluate the effect of seakeeping performance on mission-effectiveness of ship system and performance effectiveness of ship subsystems for several kinds of patrol boats which are assumed to be operated under the same mission, “Surveillance of a ship in distress at sea”.