A series of two-dimensional model tests has been conducted to study the coupling between global roll motions of a floating liquefied natural gas (FLNG) vessel and internal sloshing. The model of the FLNG is allowed to move freely in roll under the excitations of an initial heel angle, band-limited waves, and regular waves. To clarify the coupling effects, the FLNG vessel in different filling conditions is ballasted in fresh water and equivalent steel ballast weights, respectively. Time series of both the internal sloshing and the global motions of the vessel are measured. Statistical and spectral analyses have been carried out on the measured data. Sloshing oscillations in different surface modes have been observed. Asymmetry of the internal wave profile relative to still-water surface is also observed. Attempts are made to clarify the influences of the internal sloshing on the global roll motions through the comparison of the experiment results between the liquid and steel ballasting cases. The coupling phenomenon is found to be sensitive to the period and height of excitation waves. Further discussion has been made on the experiment results, and some conclusions regarding the coupling mechanism between global motions and internal sloshing are drawn based on the present study.