Researches to predict the ship motions and accelerations to be induced on a nuclear ship in waves were carried out by model tests and theoretical computations. Firstly, the responses of the ship, which advances with various speed in regular waves coming from various directions, were measured and compared with the computed results by so-called strip theory. Secondly, the model tests in irregular waves having a certain energy spectrum were carried out, and the short term distribution of the probability of the responses were estimated through the cumulative energy density of the spectrum of the responses which were derived both by the test results and the computations. Finally, the long term distribution of the responses were also estimated by using the derived short term distribution and the probability of occurrence of sea state at the northern North Atlantic Ocean.