For the vibration analysis of floating structures, fluid-structure coupled analysis method which combine the added mass matrix into FEM has been used since many years ago. However, this method has some problems. In the first problem, it is well known, preparations for the FE-model are timeconsuming works. In the second, designers are at a loss how to deliberate the result because too many eigenvalues are obtained. To solve these problems, reduced added mass matrix method was proposed, but the effect of rotation vector upon the accuracy of calculation through the reduced matrix was not discussed. In this paper, previous effect is investigated by comparing some calculations on a boxy model ship. The eigenvalue analysis is executed to the ship which is represented by FE-model, and two kinds of modal structural modification analysis are done by using the result. One is the calculation with the eigenvector in a direction, and the other 6 directions. From these calculations, it is explained that the rotation vector should be considered for the application of the reduced added mass matrix to vibration analysis of higher order modes. These calculations are compared with the experiments, and a certain inference is shown on the fluid-structure coupled vibration analysis.