Horizontal and torsional vibrations of container ships have been studied theoretically and experimentally. A theoretical calculation, in which three kinds of torsional rigidities ; St. Venant's torsional, Wagner's bending-torsional and Matsuura's shear-tosional rigidities are taken into account, has been developed and coded into the computer program. Vlasov's bi-moment is also considered regarding the warping stress. The vibration test has been carried out on a large container ship, the results of which, together with the experimental data of another container ship, have been compared with calculated frequencies and responses. The theoretical method can explain the phenomena observed on full scale ships, though future studies on the center of twist and the virtual mass are necessary to refine the method.