The present paper deals with an investigation. into the variation of the natural frequency of ship vibration in wave. As a result of their study, it has been found the natural frequency of the two node vertical vibration of cargo ship in ballast condition decrases about ten percent in sagging condition and increases about three percent in hogging of standard wave compared with that in still water by steady state calculation. The cause of variation of natural frequency is mainly due to the change of distribution of virtual added mass of water. Only the frequency modulation occurs in the free vibration of ship's hull on wave while the considerable amplitude modulation appears in the forced vibration. Some experimental verifications of the steady state calculations were carried out using the models of ship shape, but further investigation on the problem of unsteady state is required.