This paper deals with a practical method for vibration analysis of large ships. The vibration characteristics for 230, 000 DWT oil tanker are also analyzed and compared with experimental results. The results obtained are as follows. 1) In estimating the vibration characteristics of large ships, a response factor of acceleration is effectively used, which is determined by the mode shape matrix and the generalized mass. 2) Although large ships usually have lots of eigen values, the vibration modes with large response factors are relatively few and localize upon some frequency regions. 3) The higher modes with large response factors are remarkable at superstructure and afterbody of shiphull. The response factors of their modes may considerably vary with the load condition, by which little frequency changes are observed. 4) The calculated and measured natural frequencies for the lower modes show good agreements. Although the data for the higher modes are not sufficient, it can be seen that the peak frequencies on the measured resonance curves correspond to those of the typical calculated vibration modes. 5) The calculated resonance curves are similar to the measured ones over a frequency range of 500 to 600 cpm except some cases. In general, further investigation will be required about estimating the exciting force, such as surface force induced by propeller and axial force of shaft system due to main engine.