In order to prevent harmful vibration in ships at the early stage of design, anti-vibration design is required on the basis of accurate estimation methods. In the field of vibration response prediction, however, significant improvement of accuracy is not yet achieved due to the difficulty in estimating damping. It is difficult to deal with damping of ship in a theoretical manner, since the damping mechanisms are complex and related to the dissipation of energy occurring in different ways. Therefore, damping should be obtained by the experimental approach. With regard to damping factor measurement, response function is acquired from vibration measurement of actual ship by exciter test and it is processed by modal analysis method. In spite of these measurements, it is not an easy work to apply modal analysis method to ship response analysis, in particular an analysis of huge scale structural model, such as 3-dimensional finite element analysis of whole ship structure. The reason for the difficulty of response analysis by the modal analysis is because it needs a lot of skill to identify major vibration modes from various vibration modes obtained by modal analysis of large structural model. In this paper, a new identification method of damping factor is proposed to overcome the above difficulties by calculating dynamic response directly on the basis of Rayleigh's damping, [ C ] =α [ M ] +β [ K ], as well as by correlating measured data and calculated one using iterative method. This paper describes that the theoretical background of this identification method to obtain identified coefficient of damping α, β is explained firstly and secondly performance of convergence is confirmed by a simple hull girder model, and finally this method is verified by the measured data of frame structure model.