In recent years, ballast water has been blamed for a variety of marine pollution problems, particularly for transporting harmful aquatic organisms from one part of the world to another and damaging the ecosystem of the new areas. A relatively simple mechanism to control this problem is to exchange ballast water on the high seas between ports in order to remove invasive species before the ship reaches its destination. However, some issues regarding ballast exchange on the open sea need to be addressed before this operation is introduced. One of them is the sloshing of the sea water in the ballast tank. In this paper, ballast water exchange on the open sea by means of the Sequential exchange method is simulated. Irregular seaways are generated from the ISSC spectrum, and the sloshing response of the ballast water due to the roll motion of the ship is numerically computed. Ballast tanks of Panamax and Cape-size bulk carriers are examined. The result of the numerical simulation shows the occurrence of a severe sloshing at the lower part of the bottom-surface of top-side tank of the Cape-size bulk carrier. It is also proved that the danger of sloshing decreases remarkably if the course of the ship in waves is changed.