Experimental research on capsize of a box-shaped model having extraordinary large depth or draft in comparison with breadth has been carried in a model basin with wind tunnel. The object of the experiment was to confirm which term of Mathieu equation, the constant or time dependent part of the restoring force, was the main cause of the instability of non-linear oscillation. Our conclusion is that the instability due to time dependency of restoring force is difficult to appear or become large under usual conditions of ships and the main cause of capsize looks like to come from the loss of restoring force caused by the immersion of deck edge combined with heaving and drifting (or swaying) motions. The paper contains the description of the experiments, an example of calculation of nonlinear heaving of ships and the consideration on experimental results and the mechanisms of capsize with an aide of motion picture analysis.