The Authors have been investigating into the effectiveness of anti-pitching tanks in reducing pitching motion of ships. The idea is to put openings at the side of fore or aft-peak tanks to let sea water flow in and out at 90 degrees phase lead to the pitching and therefore will result in increase of damping for pitching motion. Through fundamental researches, the Authors found that the larger the water plane area of the tank the more effective the tank and that shallower the position of the openings the more effective the tank. It was also found that if the openings are attached by ducts of certain length, the tank will be more efficient. A full scale ship trial was made on a buoy tender HOKUTO of 537. 63 G. T. Fore-peak tank only was provided by under water openings. The waterplane area of the fore peak tank was 1. 7% of ship's waterplane area. The tank was also provided by two air ducts which can be blocked to make the tank inactive for comparison. Trial data at the opening of Tokyo Bay show that pitching motion was reduced by 30% to 50%. This figure almost doubles the predicted effectiveness of the tank by model experiments. Cause of this discrepancy has not yet been clarified.