Free liquid surfaces inside the cargo tanks of seagoing ships not only reduce the initial stability but also influence their sea-keeping characteristics. The loading/unloading procedure of an average LNG carrier vessel takes 18 to 24 hours; during this time, the vessel's response amplitude operators (RAO), especially the roll motion, are changing. This is ascribed to coupling effects between the moving liquid and the hull motions. Hence, the sea-keeping characteristics have to be reappraised for each case. A 138,000 cu m LNG carrier with four membrane tanks is numerically analyzed with a potential theory-based software at different filling levels and wave incident angles in frequency domain. Instead of one single resonance peak as for the solid filling case, the roll RAO of the dual mass system features two peaks, whose magnitude and position depends on the filling level of the cargo tanks. However, coupling effects due to resonant sloshing also affect longitudinal body motions. This knowledge regarding altered sea-keeping characteristics of LNG carriers due to free liquid surfaces is essential for safe offshore transfer operations.