Model tests on capsizing of a container ship running in quartering seas were performed using a radio-controlled model. The model tests were carried out for the one P-M type irregular wave and various regular waves in the 80 m square tank of the Ship Research Institute. The parameters of the experiment were the GM value, the model speed and the encounter angle with the wave for the irregular wave, and also the model speed, the encounter angle, the wave length and the wave height for the regular waves. The encounter angle was varied precisely every 10 degrees from 0° (following wave) to 90° (beam wave) by using the auto-pilot steering system in order to specify the dangerous range. The Froude number of the model speed was varied every 0.01 to clarify the critical speed for the capsizing. The wave length and wave height of the regular wave were also varied in a fine intervals to examine the tendency of the capsizing for the various waves. Among 763 runs in the irregular and regular waves, 225 capsizings were observed. From these results three characteristic features were clarified. Firstly, most capsizings occurred in the range of the encounter angle of 20° to 40°, and the range was spread to 0° to 50° as ship speed increased. Secondly, as ship speed decreased, the capsizing decreased, and there existed a critical ship speed below which the capsizing never occurred. Thirdly the direction of the capsizing was always to the leeside. On the basis of the above results, a hypothesis is presented that such capsizing may be explained fundamentally as a simple static one which occurs because the capsizing moment of the wave exceeds statically the restoring moment decreased in the quartering wave in the above dangerous range of the encounter angle. The reason why the capsizing occurs only in the high speed range and only to the leeside direction is also explained in connection with the surf-riding motion or asymmetric nonlinear surging motion with a large amplitude. The capsizing due to the parametric oscillation, which had been regarded as one of the typical mode of the dynamical capsizing, was never observed in the present experiment. Instead, a number of the new mode of the dynamical capsizing accompanied with the phenomenon of the period doubling bifurcation which was regarded as the foreboding of the chaos were observed.