Sloshing is a potential source of disturbance in liquid storage containers. Coupling effects of ship motion with fluid oscillation inside three- dimensional rectangular containers using a newly developed time domain simulation scheme were studied. Some representative studies are carried out by the authors. When the free-surface profile is involved, the three-dimensional effects became more significant. Coupled analysis predicted that the effect on the ship motion can be even up to 30% due to sloshing inside the container. The results advance the understanding of the dynamic behavior of LNG carrier in three dimensions interacting with the external hydrodynamics. This is an abstract of a paper from Proceedings of the Nineteenth (2009) International Offshore and Polar Engineering Conference (Osaka, Japan 6/21-26/2009).