The coupled vibration of torsion and horizontal flexure of a container ship was studied theoretically and experimentary. In the calculation method developed here a ship was treated as a thin-walled beam with non-uniform cross-section. The authors especially took note of the problems occurred in the calculation based on the non-uniformity of cross-section along the ship length. Haslum and the other's method was applied to the continuity of warping and the equilibrium condition of bi-moment at the points where the shear center axis is not continuous. And the effects of the couples between the horizontal shear force and the torsional moment and between the horizontal displacement of the shear center and the twisting angle were also taken into consideration. The vibration test was carried out on a ship model with non-prismatic form. The results of this investigations led to the following conclusions. The calculated results obtained through this method are in good agreement with experimental ones for lower mode vibration. And this calculation method can explain the phenomenon on a container ship in which the corresponding horizontal and torsional vibration modes have different number of nodes.