The Author has been conducting experiments to measure the added mass and added moment of inertia of ship forms for the motions through six kinds of freedom, i. e ., translations to x , y , and z direction, and rotations about x , y , and z axis. In this paper the Author states about the results of measurement of added moment of inertia about z axis, which were obtained by an impact method properely deviced for this purpose. Effect of the free sarface upon the added mass and diference between added masses for translation tolone direction and for oscillatory motion were also discussed in this paper.