The position of ship's centre of gravity is usually measured by inclining experiment. The existence of various errors is inevitable in the measured value, as the above method is not direct but indirect. The aurhors made an attempt to investigate the actual variance of the measured positions of C.G. based on sister ship groups. Samples are picked up from the defunct Japanese Navy vessels, whose trial displacements lie between 2,000t and 3,000t. According to the investigation, the following became clear; (1) Distribution function form of measured values is Gauss' normal distribution. (2) In our investigation range, 90% confident intervals of standard deviations for displacement, KG, GM are as follows; [table]