The nuclear power plant for ship must be able to endure sufficently to the severe ship motion in rough sea. Therefore, in the design of reactor, we must predict the magnitude of acceleration_ or inertia force acting on it. In this paper, we attempted to the statistical considerations on the vertical acceleration due to the pitching and heaving, and some consideration of rolling acceleration. They are as follows. 1. From the response amplitude operator of vertical acceleration that is measured at model experiment in regular wave and Neumann's spectrum of sea wave, we calculated the statistical magnitude of vertical acceleration of small fine ship, high speed cargo liner, and large oil tanker for various ship speed and wind velocity. 2. From the above data, under the following condition, we obtained the values shown in Table 4 which was regarded as the reasonable values of vertical acceleration to use in the design of reactor part of various nuclear ship. a) As the ship speed, we adopted the maximum mean speed obtained from the point of view of resistance increase only. b) As the sea condition, we adopted the fully developed sea for wind velocity U =20 m/s. c) As the statistical magnitude of acceleration, we adopted the maximum acceleration a ₀, which is the vertical acceleration that the probability which maximum vertical acceleration in 5, 000 continuous waves exceeds a ₀ is 10%. 3. The rolling acceleration at resonant condition are expressed by Ts·√ g/B only approximately such as Fig. 17. The magnitude of rolling acceleration is smaller than that of vertical acceleration by pitching and heaving, but it must be careful because of the difference of its direction.