In case of less stable ships or unstable ships such as full bodied ship, there are difficulties in conducting ordinary zigzag maneuver at a small rudder angle, say 5 degrees. In order to examine the course keeping qualities of such ships, the authors proposed a modified zigzag maneuver, which was distinguished from ordinary zigzag maneuver in the respect that the rudder angle was not equal to the switching heading angle. As an example, results of ordinary and modified zigzag tests on two full supertankers are reported and the maneuverability indices, T ^* and K ^*, obtained from them are compared with each other. In the latter half of this paper, it is described that modified zigzag maneuver is valid to obtain frequency response function of maneuverability.