To prevent or reduce the vibration of ships, it is necessary to know their natural frequencies. Hitherto the various methods of calculating the natural frequency of ship's structure have been published by many researchers ; but some of them are too complicated for preliminary design use, and the others lack in accuracy. The authors have a plan to carry out a series of vibration tests of many vessels, in order to obtain the accurate method of estimating the natural frequencies of ships for preliminary design use. And they have designed a “Vibration Exciter” for actual ships' experiments. This instrument can give any periodical force acting on an actual vessel by changing the speed of revolution of its eccentric masses. In this paper the authors report the principle and mechanism of the vibration exciter and the results of their experiments that were made on a 450 tons patrol boat. As the conclusion of these experiments, it may be said as follows; (1) It is possible to measure the natural frequency and the mode of vibration of an actual ship by making use of this “Vibration Exciter” and vibrographs. (2) It is possible to make clear the effects of weight distribution, depth of water and position of unbalanced force, the damping factors, the wave making phenomena, and other characters of the vibration of vessels by these systematic experiments. (3) The natural frequency of the patrol boat measured was 303/min. and 301/min. at respective conditions of experiments and the nodes of vibration were found at Fr. 30 and Fr. 59. The damping factor was about 7%.