In this study, large amplitude rolling motion of a ship without bilge keels in heavy regular beam seas is experimentally investigated, and a new non-linear feature of lateral ship motions in high waves is revealed. The experiments show that, in high beam waves, large amplitude rolling motion occurs not only in the frequency region of the roll resonance but also in the frequency region higher than the resonance frequency, and that jump phenomena of roll amplitudes appears near the heave natural frequency. These behaviors obtained by the experiments are investigated, and the reasons of these non-linear features are clarified as follows; the former is caused by changing encounter frequencies by large drifting velocity in high waves to make the roll-resonance region wide. The latter is caused by different drifting velocities for the same regular wave near at the heave natural frequency. A non-linear feature of heaving motion is also experimentally confirmed near heave natural frequency.