One of the principal causes of the fractures around the slot of transverse web is considered to be the effect of lateral vibration of transverse member. In this report, at first, exciting tests were carried out with the bottom trans. model of 1/4 scale actual ship in order to investigate the natural frequencies and modes in detail. Secondly, during the sea trial of 224, 841 DWT and 284, 495 DWT oil tanker, comprehensive vibration measurements of ship hull and bottom trans. were carried out and those data were used to find response characteristics and excitation sources. Finally, the theoretical analysis of natural frequency and hydrodynamic pressure for the lateral vibration of trans. member were carried out and the former results were compared with the measured values. The results obtained are as follows : (1) Calculated natural frequencies and modes except for modes in water showed considerably good agreement with those of the measurement. Natural frequency of 2 nd order in water is less than or nearly equal to one of the O-node axial vibration of shaft system. (2) Hydrodynamic pressure distribution depends on natural mode of member itself, correlation between adjacent ones and the shape of bracket and face plate. Therefore, further investigation is necessary to estimate virtual added mass. (3) Engine and propeller induced axial forces of shaft system are mainly transferred to the bottom trans. and amplified through axial vibration of shaft system and ship hull vibration.