A series of two-dimensional model tests has been carried out to study the hydrodynamic performance of a floating liquefied natural gas (FLNG) section including internal sloshing oscillations. The reference FLNG section is ballasted with fresh water and equivalent solid weights respectively, to clarify the coupling effects. In addition, five different ballasting conditions of the FLNG section were considered, to investigate the influence of filling levels and natural frequencies. Response amplitude operators (RAOs) of both motion responses and internal sloshing flows are calculated based on measured data. The inner-tank sloshing exhibits obvious effects on sway and roll motions, while little effects on heave motion. It is observed that the first mode of sloshing can significantly affect the global motions of the vessel, while that in higher modes shows little effects. The coupling effects are found to be sensitive to the filling levels of the tank and the roll frequencies of the vessel. Whether the internal sloshing amplifies or reduces the global motions is related to the difference between the first mode of sloshing frequency and the roll frequency of the vessel. The outcome of this study would offer better understanding on the coupled hydrodynamics of ship motions and sloshing flows.