To investigate the rolling motions of a flooded ship, non-linear coupled dynamics of roll and flooded water are derived using Lagrange’s principle. Decoupling the coupled equations gives a single-freedom ordinary differential equation, which can describe the rolling motion of the flooded ship. Melinkov’s method is used to predict the chaos behavior of the ship. Numerical computations are performed for three statuses of a ship model. The comparison of the results shows that the more cabins flooded, the more likely a chaos phenomenon will appear.