Abstract: The dimensionless analysis is used to deduce the expression of natural circulation flow of nu-clear installations under ocean condition and the natural circulation flow equation in the steady state. The theoretical results are consistent with the experiment values. The expression between rolling amplitude and rolling period is also been deduced by the dimensionless analysis, with the limit that the fluctuation of natural circulation flow cannot cause the response of the control rods. The relation between the rolling amplitude and period is quadratic. The theory results are also consistent with the Code results. The responses of control rods depend on the maximum rolling acceleration, and have nothing to do with the maximum rolling angular ve-locity.